Most older games will have 2-3 engine loops that they will blend between and pitch accordingly, however Forza 5 specifically was the first in the series to use granular synthesis which is a much more modern approach.
These will have a series of sound files including all the different parts of the engine, exhaust, turbo, etc. Then the software slices these files into tons of pieces in real-time and plays the slices based on the players inputs.
It’s tricky to explain without examples, so here’s a video that does a decent job of explaining how it works in music: https://youtu.be/ftDLRYnRYZQ?si=SEHOl55K7rimi4Eu
This one will still be playable offline after the fact, so its offer is way better than the games that are the focus of the Stop Killing Games movement. The games they’re targeting are completely unplayable after being killed.