Mindustry and Unciv are my main recommendations. If you like strategy stuff, you'll have a blast.
Android
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Witcheye is a good version of flappy bird, as its controlled with only one finger, with extremely difficult platforming.
Supermeat boy forever, great game all around but very difficult.
The Layton games - the DS games have been ported to Android and totally slap. As someone who never had a DS, I'm enjoying this era of the games coming to Android and being playable here.
There is a Cultist Simulator there. I don't know about Android port much, but it's a good game on PC
Peglin! It's an amazing peggle style rouge-lite
Star Traders: Frontiers. Be a space trucker/pirate/merc. Singleplayer offline space turn-based RPG Pay once only but frequent free updates. No bullshit P2W, gacha, timers etc etc.
Bonus: kill xenos scum, customize your ship load out, satisfying crew combat.
Cons: annoying RNG card minigame for spying, blockading, salvaging. Enemies esp. xenos can get very unforgiving on high difficulty setting.
Have you tried looking on itch.io? I recently got a few games from there (some are hosted on the play store but not all)
Only bad thing about browsing is that 80% of the games seem to be dating sims or visual novels (which is fine if you're looking for that, but annoying if you're not)
Edit: infidhells seems really fun, but haven't had time to play much, to use all the gameplay features it's worth playing with the sound on (though it has accessibility options for playing without audio).
Sand:box, plague inc (i recommend buying premium for both) and all games on f-droid.
Rusted Warfare is a great RTS. It has offline and online server hosting (never seen that on mobile before). It feels like I'm playing an RTS on GameRanger.
OpenTTD! https://play.google.com/store/apps/details?id=org.openttd.sdl&hl=en&gl=US
It's a FOSS version of Transport Tycoon.
SuperTuxKart I guess? It lets you to play offline, although online mode and add-ons are required internet connection.
Vampire Survivors, 20 minutes till dawn, geometry dash. Space cadet for android - available on github, super meat boy forever.
Soul Knight is my go to
My go-to for like 5 years has been StudyGE. It's just a Geography game, but for the rare ocassions where I have nothing else to do and I don't have Internet in my phone I go to it.
There's also the trivia game Dilemo, though that is in Spanish and I think it might have some ads, it's just that it serves them so infrequently that I'm not even sure if I hallucinated them. Like I think it only serves ads when you have been playing it for long-ass sessions, and I usually only play for a few minutes, so.
The Room is a fantastic series of lovely 3D puzzle box and room games! I really enjoy them :3
Papers Please is what I mostly play on my phone on aeroplanes (if I haven't packed the steam deck). A classic that works well on mobile
Also, Mini Metro (and the rest of the series) is great.
The Bonfire: Forsaken Lands
https://play.google.com/store/apps/details?id=com.xigmagames.thebonfireFL&hl=en&gl=US
This is one of the few (not including humble bundles from may moons ago) non-free to play games I have. Great game.
If you like managing games, like manage a fantasy town, or manage a gaming development software house, or an airport, or a restaurant, or a SPA, then I'd suggest Kairosoft. Their games are the last bastion of old school pixel graphics offline games with no DLC whatsoever.
https://play.google.com/store/apps/dev?id=7710637777904825280&hl=it&gl=US
Templar Battleforce.
Magic Research
It's an paid offline idle game without ads and a nice progression. Also has a demo.
https://play.google.com/store/apps/details?id=com.mcolotto.magicresearchfull
Every game is a no ads game with blockada :)
Just saw your other comment about traveling, my go to game is Pirates Outlaws, cost a bit money but is offline ad free and very fun. I put a lot of flight hours with it
slice and dice
PewPew live and PewPew 2
Chiki's Chase is a great game.