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.