AMD and Intel aren't (or weren't at least) shipping cutting edge GPU tech so there's less need to look to them for ideas and shortcuts anyway.
Well I used to work at Google and the open source parts are a tiny, tiny fraction of their actual codebase. Windows isn't really open source at all, although in recent years a few utilities have been opened up - note, only after Windows stopped being so important to Microsoft.
And as for macOS. Well. You can download and read some code, sometimes. Good luck trying to actually build it or do anything useful with it at all. You'll find that it's (a) completely undocumented and (b) all depends on internal stuff you don't have access to. An exception is WebKit.
Well I used to work at Google and the open source parts are a tiny, tiny fraction of their actual codebase. Windows isn't really open source at all, although in recent years a few utilities have been opened up - note, only after Windows stopped being so important to Microsoft.
And as for macOS. Well. You can download and read some code, sometimes. Good luck trying to actually build it or do anything useful with it at all. You'll find that it's (a) completely undocumented and (b) all depends on internal stuff you don't have access to. An exception is WebKit.