While Alien was shot on 35mm negative film, what defines it and other films like it is that they were shot with an anamorphic lens in anamorphic widescreen, commonly known as CinemaScope[1] using the full 3:2 35mm frame rather than cropping. When the prints are projected (effectively decoded through another anamorphic lens), the aspect ratio becomes 2.39:1. Alien also had blowup 70mm prints with a less common 2.20:1 aspect ratio.
Nope, all four were shot on 35mm, but the latter three were transferred to videotape for editing, while TOS was cut on 35mm negative, so it could be “remastered” from the film more easily.
For TNG, they had to rescan and edit, and recompose VFX, and recreate some of the VFX in CG, which was much more laborious, than the process for TOS, which, in addition to be edited on film, had more practical effects and simpler other effects (which the remaster was used as an opportunity to upgrade, in some cases.)
AFAIK, cost for expected return is why they didn’t do for DS9 and VOY what they did for TNG.
How was ENT shot? Personally, I'd actually rather watch ENT than VOY, though I have to skip that awful intro song.