Garen is a bruiser. He excels in splitpushing, as he has immense waveclear, can take jungle camps easily (E deals 150% damage to jungle camps), dive weaker opponents, and has very threatening flanks with his one shot potential and high MS. Your goal with Garen is to use your MS and waveclear to gain immense amounts of tempo. I know that's kind of a high elo buzzword that's tossed around, but if you've ever gone against a Garen and had him just kind of roam all over the map and take all your jungler's camps, you know exactly what I'm talking about. This is one of the main reasons Garen is immensely powerful in Solo-Q: As long as his fundementals are solid, he will exert an immense amount of pressure on the map: The support, mid laner, and top laner will all feel like they need to help protect the jungle camps you steal, the top laner will feel like they need to protect the tower after you push in a couple waves, all the while you've used your surperior MS to roam and assasinate the enemy bot laner. While his pressure does fall off somewhat late game due to people starting to build defensive, that usually won't matter as hopefully you've done enough damage on the map that your whole team is a couple drakes and a couple thousand gold up. This pressure will cause teams to crack, as no one on the enemy team is communicating effectively, and they'll just fall under the pressure, even in Challenger/LCS. There's a famous clip in the Garen community of Doublelift just genuinely not knowing how to deal with Garen and saying he's a good high elo champ if you want to check that out: https://www.reddit.com/r/leagueoflegends/comments/w1lyxy/doublelift_learns_about_the_power_of_high_elo/

A good way to figure out what a champions identity is is to ask yourself: "What does the perfect game on this champion look like?"

To me, this is what that game looks like:

I get a lead in the laning phase, either by my opponent not respecting my large lethal range post 6 or using my sustain to force an attrition war and make my opponent recall.
I then start perma powerfarming by shoving the wave uncontested and farming a jungle camp. I'll be level 15 while the enemy top is 12 and the enemy jg is 10.
I then start splitpushing in the lane away from objectives. Since I have such a high level lead, it doesn't matter what the enemy top laner does: I will dive him if left to my own devices. This forces 2 people to come stop me, and when they try, I use my 500 MS to simply run to the other side of the map, get a flank off, and win a numbers advantage teamfight. Eventually our team will have so many objectives that we'll just win by pure statcheck from drakes or map pressure from us exposing their base by taking towers/baron/rift.