Is it necessary to build and link windows games with msvc? Pretend I know nothing about the subject.

Large code bases built and linked with open source toolchains have solved this with, for example, thinlto. And by "large" I mean orders of magnitude larger than the mentioned game.

Not necessary, but definitely the path of least resistance for developing Windows games, and AFAIK MSVC is required for developing Xbox games. Commercial closed source middleware is sometimes distributed as compiled C++ static link libraries plus headers without being able to recompile yourself.

You can use clang for windows builds. It’s what I use for my games. However, I assume that most popular engines might not compile on clang+windows for random reasons.

Genuine question, what code base is 100 million lines of code (I think that is the meaning an order of magnitude bigger that 11 million LOC). I can not comprehend how that much code is anything but dead weight saddled on ‘generations’ of a business’s employees.

If it helps, even by 10 million LOC (assumedly most of which isn't unused legacy cruft) you're starting to dig deeply into templates and codegen instead of actual manual coding. I'm sure with all this AI stuff that will only get larger and larger.

But yeah, outside of some of the largest of FAANG software, there aren't a lot of codebases that can truly justify 100m lines of code. I'm sure GTA VI is over 100m lines but can be cut down to 10m if that was a priority (it never is)