Which of these are available on Android?

ScummVM supports many many games. Some people might think it's an emulator, but it's actually a collection of reverse engineered game engines.

eDuke32 and other Build Engine source ports

https://veloren.net/ is a voxel MMO and it is already very playable

Sauerbraten

Barotrauma is source available (no commercial, public use, restricted for game modding and contributions) Librerama is dumbways to die style minigames

Unciv is pretty cool and portable.

KeeperRL is a lighter Dwarf-Fortress like game where you play as a Monster making a Dungeon.

When I played open source games around 15 years ago, these were all the same games mentioned. There doesn't seem to be a lot of progress

xevil

My favorite: cataclysm dda, flare rpg, oolite.

Beyond All Reason is a RTS made with Spring Engine that's an open source implementation of Total Annhilation.

Taisei Project for Touhou and other 2D bullet-hell shooter fans (I recommend installing it instead of playing online).

https://2009scape.org

!2009scape@kbin.social

ClassicUO is a nice open source client for Ultima Online. Need to have the game assets.

zero-k is really good rts. Its similiar to supreme commander

Warfork is my favorite