I mean, if you want to see some games’ source code you don’t have to rely on piracy. As other people have already said, there are open source games, some developers of older games have officially released the source code (notably VVVVVV, doom, and also quake iirc), some devs have released important part of their source code (e g the entire inputs handling code of Celeste).
Additionally, the vast majority of all Unreal Engine games’ engine code, including huge AAAs like Fortnite, is in Unreal Engine (duh), which is open source source-available.