(written from a Production point of view)
National Amusements, Inc., often simply abbreviated to NAI, was founded in 1936 by Michael Redstone as the Northeast Theatre Corporation and has since gone on to become one of the world's larger media companies with more than 1,500 movie screens and "controlling interests" (an industry euphemism for de facto ownership) in CBS Corporation and Viacom, the holding company of Paramount Pictures.
As the conglomerate is fully owned by "media mogul" Sumner Redstone (son of founder Michael, and until 2016 also the co-Chairman of both Viacom and CBS) and his daughter Shari, the Redstone family is presently the de facto legal majority owner of the entire official Star Trek franchise ever since "old" Viacom acquired the franchise in 1994 (Redstone had acquired Viacom previously, in 1987). The position of identifiable individual legal majority ownership of Star Trek had previously been held by the Charles Bluhdorn family (1967-1994), and before that by Lucille Ball (1964-1967; Gene Roddenberry lost all rights and title to his creation the moment he sold his Star Trek is...-pitch to Ball's company, Desilu Studios, in April 1964 ). While the 2005 split resulted in the formal ownership transfer of the Star Trek franchise from Paramount to CBS, the ownership position of the Redstone family remained unchanged.
The 2005 split of Viacom
It was Sumner Redstone, together with his trusted subordinate Les Moonves (who was embroiled in a power struggle with then-fellow Viacom CEO Tom Freston, who was shortly after the split ousted by Redstone personally ), who had engineered the Board-approved (on 14 June 2005) "old" Viacom split into CBS Corporation and "new" Viacom, which was publicly announced by Redstone in person from the balcony of the New York Stock Exchange on 3 January 2006.  The split became effective on 17 January 2006 and constituted in essence the undoing of the merger between Viacom and CBS Broadcasting that Redstone had acquired in 2000. His decision to do so – purportedly motivated by Redstone's desire to streamline future corporate decision-making,  and to cater to his desire to "unlock value" by boosting stock value and earning potential of the at that time faster growing soon-to-be "new" Viacom,   – took the entire industry by surprise. It flew in the face of the general industry trend at the time of market(share) concentration through acquisitions and mergers, most conspicuously exemplified by the contemporary activities undertaken by The Walt Disney Company to achieve such.
The split did not quite work out the way as Redstone had envisioned. Almost immediately after the split, stock prices of the new Viacom took a nosedive from which it has never fully recovered    (Restone used the stock dip as the official public reason to fire Freston).  This dip was only aggravated in the mid-2010s when – after a short-lived recovery – market value nosedived yet again because its main holding, Paramount Pictures, found itself faced with severe financial adversities, brought on by an almost unbroken string of financially disappointing film productions, including Star Trek Beyond,   , and which became one of the main reasons for the cancellation of its intended follow-up. It was CBS which, after it too experienced an initial stock price fall,  recovered strongly under the vigorous leadership of Moonves – during which the Star Trek television franchise was revived with Star Trek: Discovery – to become the far more successful and profitable one, effectively turning the tables on new Viacom, a situation Moonves came to cynically exploit later on.
Like Disney is currently doing (especially after the 2018 acquisition of 21st Century Fox), the National Amusements conglomerate is actually operating on the outermost edge of what is legally allowed. In 1948, the United States Supreme Court had its anti-trust ruling leveled against the motion picture industry – most ironically against Paramount Pictures in particular, being singled out by the Court as proxy for the entire industry – specifically aimed at the time to break the top-to-bottom vertical hold individual studios hitherto had on the entirety of the industry, otherwise known as the traditional "Hollywood Studio System" (see also Desilu Studios and Universal Studios in this regard). With full control over both production and dissemination of their movie and television productions, the conglomerate as presently organized – even after the 2005 split – is partially negating the intent lawmakers had in mind back in 1948.
Leadership change and remerger
After her father was forced to step down in 2016, officially for age reasons, and she became his successor, Shari Redstone – said to consider the Star Trek franchise as a "family heirloom", and who has never been in favor of her father's 2005 split decision to begin with, but as then minority shareholder was impotent to prevent the move at the time – has been trying to remerge the two entities ever since. However, she immediately found herself embroiled in a vicious power struggle with both CBS CEO Les Moonves, co-architect of the split, and (new) Viacom CEO Philippe Dauman vehemently opposing the proposition, both of them tightly holding on to their personal fiefdoms. Additionally, neither man had been willing to serve under the other in case of a remerger, as both men were bitter rivals as well, because each had expected to take over Sumner Redstone's position as Chairman of the Board of NAI upon the latter's retirement instead of in the mean time estranged daughter Shari.    She eventually reconciled with her father, succeeding him after all, which came as a nasty and unexpected surprise to both Moonves and Dauman. It turned out that Sumner Redstone's "streamline future corporate decision-making" motivation for the break-up (old) Viacom was in effect corporate jargon for his desire to tighten his grip on his holdings by playing both CEOs off against each other – a classic case of "divide and conquer". It is debatable if either man had ever been considered in earnest by Redstone at all.
Shari Redstone already managed to successfully oust Dauman as early as 2016 after a very unsavory power struggle complete with lawsuits and counter-lawsuits during which Dauman overplayed his hand,  which effectively ended his Hollywood career. Moonves on the other hand, learned from Dauman's mistake of only using personal, presumed hear-say promises and assurances made in private, as arguments in court (Sumner Redstone had been savvy enough not to commit anything legal on paper – further proof that his "divide and conquer" approach had always been his initial intent), and decided to play another hand, using business arguments instead. Redstone therefore found Moonves an even tougher nut to crack when he too took her to court in May 2018, seeking to diminish her influence over CBS by actually arguing that Viacom’s struggling properties would damage a reunited company.  However, less than four months later the struggle was unexpectedly and definitively decided in Redstone's favor when Moonves was forced to leave CBS on 9 September 2018 in the wake of the #Me Too movement – spearheaded by among others Robin Lefler performer Ashley Judd  – controversies that he found himself in as of July that year when The New Yorker broke the news of his alleged sexual misconduct  – actually also unofficially part of the reasons why her father had to step down, as he too had not been entirely free of similar accusations. Part of the severance agreement however, was that Redstone had to cease her remerging efforts for at least two years. But while this at first glance might have looked like a concession on her part, it actually improved Redstone's chances of her mission to succeed, as it afforded her the time to purge the Board from Moonves loyalists, as is customary in such circumstances in not only Hollywood, but in other industries as well.   
Indeed, by early 2019, tentative steps were underway at CBS to effect Redstone's ultimate goal to restructure her company's holdings, which included the replacement of much of CBS' leadership.   If successful, this will for the Star Trek intellectual properties – split up between the two entities in the 2005 "divorce settlement" – mean a "reunification" of the television and film franchises. On 19 July 2019 Trekcore reported that Star Trek had even become part of the remerger motivation, citing a Hollywood Reporter article, which speculated the "reunification" to take place on 8 August 2019 at the quarterly corporate earnings reports, and that Shari Redstone's mission was close to succeeding.  It did not materialize on that date though, but a major stumbling block in regard to who should head the yet to remerged entity was removed, as the by Redstone favored Viacom CEO Robert Bakish was decided upon as such by both respective boards (even though the CBS board represented the much larger entity),  and beating out CBS head Joseph Ianniello, the interim replacement for Moonves, but whose position was still that of a temporary CEO only.   Not only that, but Ianniello was tainted by his known association with Moonves. 
Nonetheless, one week after the corporate earnings reports, on 13 August 2019, both companies made the official announcement that the remerger was in effect decided upon as well with Bakish formally announced as the overall CEO of the remerged and newly named ViacomCBS Inc. entity and with Ianniello's position officially elevated to that of permanent CEO of the CBS part as either a consolation price or token gesture,   – though that turned out to be very short-lived, as he not long afterwards decided (or was forced) to leave the company after all, effective March 2020,  resulting in the removal of the last vestiges of the Moonves-era from the conglomerate. Actually validating the July Trekcore report, both Star Trek and her franchise sibling Mission: Impossible were as the only ones in the official CBS press release specifically mentioned by name with the statement that the remerger "reunites fan-favorite franchises such as Star Trek and Mission: Impossible".  But while the remerger has in essence all the hallmarks of the resurrection of "old" Viacom, there is one major difference; Paramount Pictures is not getting back the television division it had lost in the 2005 split. This might seem like a hindrance to a "reunified" Star Trek, but the conglomerate has apparently foreseen that when it already instituted an overall "Star Trek Global Franchise Management" in 2018,  made formal in May 2019 in anticipation of the impending remerger. 
Incidentally, and taking her cue from Disney, Shari Redstone's ambitions do not end when the remerger is effected, as she plans to embark on a Disney-like acquisition course herself afterwards, having her sights set on such companies as the film and television studio Sony Pictures Entertainment and the non-fiction TV company Discovery Communications (owner of the Discovery Channel – Disney owns the counterparts National Geographic Channel in full and The History Channel in part).