You’d be surprised at the amount of people at Microsoft that their entire career have been at Microsoft (pre-git-creation) that never used Git. Git is relatively new (2005) but source control systems are not.
I believe it. If you are a die-hard Microsoft person, your view of computing would be radically different from even the average developer today, let alone devs who are used to using FOSS.
Turn it around: If I were to apply for a job at Microsoft, they would probably find that my not using Windows for over twenty years is a gap on my CV (not one I would care to fill, mind).
It would very much depend on the team. There's no shortage of those that ship products for macOS and Linux, and sometimes that can even be the dominant platform.
Yes? If its in your field, like a webdev who has never touched Wordpress, it can be surprising. An automated tester who has never tried containers also has a problem.
These are young industries. So most hiring teams expect that you take the time to learn new technologies as they become established.