software fallback, hardware accelleration, how does it work

Hello together,

right now I do have a specific use case for Mesa 3d in mind, but I'm not exactly sure how or if it would actually work.
Maybe you can check some assumptions I've made in my mind.

Mesa provides software only rendering on Windows. Or does it use the drivers of the IHV if installed?
On Linux (Unix in general?) it's also possible to get hardware acceleration using Mesa. That's what Gallium and the like is for, correct?

If I want to use hardware acceleration on Linux, do I need to define it while compiling or does it choose whatever is there and if no alternative is found uses software rendering?
E.g. could I link to mesa statically and thus would be able to use OpenGL even if I'm not certain that mesa is installed? Of course for open source software making it a dependency would certainly be the most appropriate step.
But I'm not sure if that would also apply to commercial software. Or maybe even just software that's supposed to just download and run without any installation step.

I would really appreciate your help and thoughts.

Best regards,
Christoph