What provides the gravity for the ships in the Alien movies?

personally I think its artificial gravity but the whole series falls under a big fallacy called the "world war 2 fighter jet analogy" which is a misconception that flying a spaceship in space is like flying a conventional aircraft and this was first predominantly seen in Star Wars back in the 70s where the Rebel Alliance X Wings and the Imperial Tie Fighters (both Military Spacecrafts) were fighting and pulling off stunts and maneuvers as if they were World War 2 aircrafts engaged in dog fights, in reality space flight is much more difficult due to the lack of air and it would take a powerful boost to make a spacecraft turn sideways and even then it would take more than a few seconds, Aliens also fell under the fallacy because if you noticed the Marine Dropship "fell down" from under the Sulaco when the release was triggered by Ferro (the Marine Pilot) which makes no sense since there is no gravity and in real life it would just float upon release and the Dropship would have to make a carefully calculated path around the planet to enter it, also you may have noticed that in the Dropship Ripley and all the Marines were not floating around or anything as evidenced by Hudson standing up and walking normally during his "Ultimate Badass" rant.

so honestly I don't know of any explanation of any Artificial Gravity system because its fairly obvious the original designers never put much thought into it, but as the series gets more realistic I could be wrong but I wouldn't be surprised if a explanation is retconned into the canon.

The Force

Its called "suspension of disbelief".

No, they haven't, and they probably never will. It would take too much time to explain it, in a movie, so it's just assumed that artificial gravity exists.

sloppy scriptwriting