Do keys from MacGameStore activate on Steam for Windows?

for example https://www.macgamestore.com/product/10476/Cities-Skylines-Campus/

Avaiable for Linux and Windows but click on the banner to go to mac

Yes, they do. There are only a very few select exceptions, and they are usually games released in the early 2010s.
But if you are so worried, just buy the same game at WinGameStore, they are the same store.

Yes, with very rare exceptions.

Steam key is Steam key. A Steam key for Mac will activate for Windows, except where it explicitly says otherwise, like talgaby said. Done that in the past a few times where MacGameStore was significantly cheaper. Nowadays most of the games there have the same price as WinGameStore, so it has been years since my last purchase.

one of my fav. stores. macgamestore as well as wingamestore deliver steamkeys and as long as there is not a key only for mac it should work without any problem.

I've bought stuff on MacGameStore before and and it has always worked on my Windows PC. There are a few exceptions - as far as I know they are all Call of Duty games and clearly have separate Mac versions for sale in the Steam store.