In TOS, I thought they had a belt. TBH, Trek kind of becomes impractical with many devices somewhat magically appearing in many cases. I mean do uniforms even have pockets?

So before ENT, Roddenberry would say that Starfleet uniforms have no pockets,zippers, or buttons as there's no need for them.

Yes, in TOS they did wore a belt. I believe in TNG era it was more a velcro or something we never got an explanation.

On that front, Roddenberry was an idiot. That said, there is definitely an element of gender equality on the no pockets policy... Although, I think the balance should have been toward pockets for women and not no buttons, pockets, or zippers.

At least Trek doesn't have crypto... yet.

Early TOS uniform features a wide leather-ish belt, later is more of the Velcro approach.

I don’t see how they could have anything attached to those early TNG spandex jobbies.

Yeah, I have to agree with Fandyllic on Roddenberry's stance towards pockets--just because its the future doesn't mean there isn't still a need for them, dude! As demonstrated in every Trek show ever, there's still a need to carry things after all! :-P

Honestly, I think it was just a case of him trying to reinvent a wheel that didn't actually need reinventing.

To answer the original question though, there's not really a good explanation for it. They just sort of magically appear whenever there was a need for it, then magically disappear again when they weren't. From the production standpoint, they almost certainly were just boxes the team velcroed onto the costumes whenever the scene called for it, probably with the intent of making it look like it was this awesome futuristic thing, but I always thought it looked cheap, clunky, and bulky and I kept wondering why they didn't just use conventional pockets instead.

Nano-bot electro-adhesive!

I think some had straps. Others seem to have used velcro.

Thanks everyone

They have Lucasarts Adventures style pockets.

I recently learned some on-screen uniforms do actually have pockets in PIC: https://www.youtube.com/watch?v=ftj-0uDD5V4&t=390s