Why 20Gb game would run in 8GB RAM ?

Yes, the application needs to be loaded into RAM.

Yes, a 20GB game can run in 8GB of RAM.

The mistake you're making—which is a natural one—is that if a game is 20GB, the application is 20GB.

It is not. Of that 20GB, a surprisingly small amount is application code. Most of that 20GB is music, models, textures, graphics, and so forth. Not all of that needs to be in RAM at once.

If you're playing Skyrim and you're in your house in Whiterun, the graphics for the Shrine of Azura do not need to be in memory. When you move from one location to another, you will sometimes see a loading screen as unneeded graphics are flushed out of RAM and the new graphics are loaded into RAM from disk.

Also, one game may include several bits of application code or several libraries that do different functions. They are loaded into RAM when they are needed, and purged when they are not.