Ads upon ads upon ads
And we need a store with actual quality apps (with no micro transactions, and being reviewed by people who care about usability), because certainly Google Play is not a good source for them.
Fdroid apps CAN have microtransactions I believe, and they’re not curated either, but overall, the quality is much better than Play Store and most apps are free and open source.
Alternatively, for games in particular, Apple Arcade is great, but it’s a subscription (cheap if combined with Music, TV and iCloud storage, otherwise several euros per month). The games aren’t allowed to have MTX or ads if they want to be on Arcade. But of course that requires moving to iOS (where the app store is much better than Play Store, BUT you don’t get alternatives like Fdroid on Android.)
I use a huge amount of open source f-droid apps, like my keyboard, my mail app, the one i’m typing on right now, SD maid 2, (Revanced). in my opinion these apps are just better. not only because i can trust them, also because they simply do what i downloaded them for, in the most efficient way possible.