My understanding is that TOS and TNG were shot on film and the 1080p versions aren't upscaled but are actually rescanned from the film. TNG supposedly had to basically be re-edited from the original footage after rescanning, but not sure about TOS.

That method wouldn't be possible for DS9 or Voyager as both of those shows were shot digitally at 480p. I remember hearing that there's been some community work that upscales DS9 to 1080p using neural networks, but haven't really been able to find anything concrete about that.