For being a braindead champ, Garen requires a lot of micro-management
Since he sucks so hard and gets outtraded by all other juggernauts, by some tanks, By Yasuo, Olaf, Pantheon, Fiora, Irelia, Jax, Renekton probably Camille too, he has no gap closers for the ranged match-ups.
I watched Riste a little bit(reached Challenger on Garen), so you have to know when to stop a trade, give up CS, stay away and wait for the regen, how to itemize(sometimes this guy goes armor 1st, sometimes Cleaver, rarely Trinity and almost never lethality), what runes to take(Phase Rush/Glacier Augument are options over Grasp vs certain match-ups).
No joke, but even with Garen being braindead, actually winning lane on this champ requires a lot of micromanagement. And he isn't godtier splitpusher/tower taker and falls off hard in teamfights.
His villain mechanic for shutting down fed carries and the crazy lv11 regen it's why Garen is even playable.
He is also decent at coming back from behind if your team helps, you just press R in teamfights on the mage or ADC.