Almost all, big game publishers have a section in their website which is called "free games". These free games are mostly older versions of modern games, or games that were once very popular but these days have been overshadowed by others. As an example, you can find GTA 1 and 2 (and a DLC/add-on for GTA 1) for free in the developer Rockstar's official site. Similarly, you will find many free games in EA origin store and Steam. Just go ahead and explore these games. When you decide on a game, then check its minimum system requirements https://sysrqmts.com/games/z1-battle-royale to make sure that it will work quickly and well on your computer.
Steam's been mentioned, and it's a good site for many of the more popular games for PC, but it has one drawback that I don't care for- DRM (Digital Rights Management, the software that dictates how and where you can install programs or listen to music).