Have a look on F-Droid, there's a whole heap of open source games on there

Any genres in mind? Shattered Pixel Dungeon is a pretty fun one, IMO. I like Idle Fantasy for its vaguely RuneScape feel (though it's largely text-based, so don't expect much graphics) while letting you queue actions up for hours of letting the game run idly. 2048 is classic. Luanti (similar to Minecraft) is available on mobile but such a huge battery drain I haven't played it much TBH. And UnCiv will let you play a game similar to Civilization. I have all those on e/OS (de-Googled Android fork) and as I recall they are all open-source and ad free.

Mindustry has an android app. I had a bit of fun with it until it kept breaking on a particular level on my burner/travel phone. Not sure if the bug was because it was an old phone or what though.

Fruity game on F-Droid is good and simple.

Luanti is a great one.

Lichess

https://f-droid.org/packages/

For a more basic time killer, I personally enjoy Feudal Tactics. Basic gameplay of using units to try and defeat all enemies on a randomly generated hexagon grid map by capturing their tiles.

SuperTuxKart?

Sometimes it's fun to smash some AI in the game Feudal Tactics (or get wiped out). Not a thorough game, but it's surprisingly repeatable if you'll get along with its vibe.

And generally F-Droid.

Aurora Store, a way to access Google Play's Store (explanation likely slightly wrong), has a filter for "free" and "advertisement free". Do a search for "open source games", add those filters and then check the web pages for any games that look interesting. Similar filters do not appear in the official Google Play Store.