the game must run atlist on windows right? 

because i have no idea how to make it run on other platforms

I believe you can make it on which ever platform you want. Based off the FAQs, it doesn't even have to be a video game.

• Can I make a tabletop, card, or analog game?

Yes! Jammers have made these games before and itch.io is very open to different game media!

• Can I make a mobile game?

Mobile games are tricky to distribute through itch.io but we won’t discourage you from submitting them! At the moment, iOS can only be distributed through the app store. You can, however, upload Android application packages.

^ See above, thanks for responding! Looks like maybe it's time to clarify the FAQ so that this is clearer since this was also answered here: https://itch.io/jam/my-first-game-jam-summer-2021/topic/1471691/type-of-build