No jokes, im actually curious on how the code works

hello! college kid who majors in video game design here i started developing things on my own and it got me thinking on how the code for such a huge game could possibly be organised so well as well as managed between so many people (im just guessing at least 100 people but thats just a random number really) and how different versions of it are managed between all these people because obviously people are all working on their own additions and what not. im guessing some form of github right? also guessing you cant go too in depth because of confidentiality agreements...

thanks :^)