I know that the game Alien: Isolation is between Alien and Aliens. However, it is from a long time later than those films, so I’m thinking it might have big connections to the prequels or something. What should I watch before it?
Isolation has no continuity with the prequels as far as I can remember. My advice to you is this:
Watch Alien, the theatrical cut.
Play Alien: Isolation.
Read Alien: Out of the Shadows, set after Isolation but before Aliens.
Read Alien: River of Pain, set after Out of the Shadows and acts as a direct precursor to Aliens.
Watch Aliens, the director's cut.
Watch Alien 3, either the theatrical or extended cut.
Play Aliens: Colonial Marines, stated as canon by 20th Century Fox.
Watch Alien Resurrection, either the theatrical or extended cut.
Read Alien: Sea of Sorrows, set after the last two movies and is a sequel to the events of Out of the Shadows.
Do that, in that order, and you have what is basically an accurate self-contained continuity/"canon" as established by 20th Century Fox (AFAIK) that relies on only the movies and the selected tie-in media and omits everything else that has been released or published before and after. I personally regard that as the "official" canon.
The first two movies should be enough. I can't think of anything in the game that isn't covered by them. Come to think of it, only the first movie would be required. You just need to roughly understand what happened to Ripley and co. and that corporations in the future are not nice people.
Honestly I had a lot of fun going in blind. The game explains enough that you never really feel lost.