Android does have a cost. While the OS itself is free, any manufacturer that wants to put Play Store, which is almost every company outside China, needs to pay Google a license fee, which effectively pays for Android. Of course there are also ads everywhere in Android and Android apps that helps pay the bill.
Wow, so the only OSs with no money in them are the FOSS ones. Makes sense, though.
(No, at this point Android hardly counts as FOSS anymore.)
There are really any ads in Android itself, even with Google Apps installed. Which, by the way, you don't need to use even if they are installed (except, for example, Chrome to get a different app store or whatnot-, just like a fresh Windows install needing to use Edge to get Firefox or Chrome).
And it's still miles easier to get Android to switch default apps and also respect your choices, than to get Windows to allow you to switch default apps and then shut the fuck about their crap.