"What design apps are the Microsoft designers using on a Mac that they can't use on a Windows machine? (Is it Mac-only Sketch?)"

The main designer tools, as far as I am aware, are bundled in the Adobe suite and that works on windows, as well as Mac.

But there are tons of other designer apps, that are Mac only - but I would assume it is mainly a designer thing to have Macs - they have been succesfully marketed (and to some excent were designed) for creative folks.