It's not common, but there are a few good, free, and ad-free games:
- Unciv: basically, Civilization V with pixel graphics. It's also open-source and you can download it from its author's GitHub.
- Underhand: very good esoteric card game. It's almost like a Cultist Simulator lite.
- Vroomies: I don't generally like racing games but this one is just perfect for mobile.
- Berserk Survivor: absurdly good Berserk game that you can get for free on Itchio
- Shattered Pixel Dungeon: one of my favourite roguelikes ever.
- Cataclysm Bright Nights: it's the best fork of Cataclysm DDA and it's well adapted to Android.