I would recommend Unciv. It's a Civ5 clone that is surprisingly feature filled. Written in Java so you can play on desktop or Android.

There is also FreeCiv but it tries to clone Civ2 which has dated mechanics by todays standards (but a lot of fans still I guess).

I would rate all of these, as worth a try:

Shattered Pixel Dungeon

Cube2:Sauerbraten

SGT-Puzzles

Andor's Trail

AssaultCube

Minetest

Neverball/Neverputt

PowderToy

0ad

Fillets-ng

Anuto TD

Xmoto/Bloboats

Flightgear

Kobo Deluxe

Enigma (oxyd)

LiquidWar5

H-Craft Championship

Numpty Physics

Wesnoth

The Dark mod

Have completed SuperTuxKart, BlobWars 1&2, Flare, Frozen Bubble, Hex-a-Hop, Holotz's Castle, SearchAndRescue II, Alex the Alligator, Project:Starfighter, Stormbaan Coureur, Trigger, etc

Used to enjoy Red Eclipse1.6, before it was retired.

I'm looking for the name of a FLOSS Quake1-mod, puzzle game, that was about placing gravity points to curve a stream of particles around the level, and eventually into the goal target. (May have used irrlicht) If anyone knows the name of this one, please let me know, it is my white-whale of games.

Can find details about most of the above games here:

https://libregamewiki.org/List_of_games

FLOSS gaming is excellent, thanks to all these devs and asset creators.

That’s a secret