Almost all commercial software are advertising their manufacturers other stuff. Ever installed a graphics card driver? How many ads does that show for games and software that can run on the card? Steam also pushes sales by pop-up advertisements.
Google has been fined multiple times because they went much further than just advertising their own stuff.
Can you disable ads on an ad platform with a single setting? Didn't think so.
So no, Windows 11 is not an ad platform, not even close.
I don't see an ad in my graphics driver. Even update notifications are optional.
When I open steam, it is because I'm thinking of gaming and most often, I'm hoping for a game deal popup.
Though, I'd love to not require opening Steam when I just want to play a game.
And that's why I'm willing to forego regional pricing and pay almost 4x for GoG games, at least for games I feel like I will be playing for a long time.