It varies significantly depending on the company based on my experience. At my current place, I wouldn't be surprised if Macs were the default type of computer for most staff, and even the execs probably used them as a result. At the job before that it was majority Windows PCs, for both management and developers.
And it's been that way in all of them. Some have mostly Windows PCs for everyone, some have Macs for everyone, occasionally you'll see the odd person using the other kind of machine if they're really picky/need it (lots of the time, it's the designer being the only one with a Mac in a Windows only company).
Don't think I ever saw someone use Linux as an OS outside of server management.