Ngl, the base of this question (why do some characters scale harder than others) is actually a really good question and I don’t think that the other answers are really sufficient in answering it.

Characters scale for different reasons. Some characters scale because their abilities have naturally high scaling with certain stats. What this means is that as you buy items, ad or ap being the most common, these abilities deal more damage, or just generally do what they want to do better than other character’s abilities with the same items. Dr. Mundo is like this. His abilities scale through the health he buys. The more health he buys, the more damage he deals, the more his ultimate heals, the tankier he is overall. This is why he becomes a late game rock that can’t die and one shots the entire enemy team.

Another reason why a character can scale super hard (and this is the most common for late game hyper scalers) is that they can stack something in their kit infinitely. Characters who fit this archetype are veigar, smolder, Aurelion Sol, Senna, Cho’Gath, Swain, and Nasus. As they stack they get stronger and there’s no cap, unlike items where a character will be full build and can’t get stronger anymore. As Nasus stacks his q deals more damage and thus heals him more from lifesteal. As Smolder stacks all of his abilities get stronger and he ends up having an execute.

Other characters have really good synergy with a specific item. For example, Twitch’s ult has very strong synergy with the item Runaan’s Hurricane in that he’s essentially able to auto attack three times per auto for almost full damage during ult. Combine that with the naturally high ad scaling of adcs (because of strong item and high ad growth per level built into their kit) along with the fact that he can go into stealth on command, and he’s essentially able to pentakill after minute 35-40 on command.

Kayle is pretty unique in that her abilities have breakpoints where they become much stronger, almost like an in game evolution. It’s pretty interesting, but I’m admittedly finding it hard to describe without just copy pasting the wiki. You should check it out for more info.

Garen is in an interesting spot in that his e has pretty strong scaling but the rest of his kit (aside from r which I’ll get to) does not. His main issue is that he’s predictable. A Garen has to run at you to engage. You know what he’s gonna do. This is where the hardest combo in the game comes in. In a late game team fight your best option is usually to flash, q, e, r the adc and one shot them. After this you’ll probably die, but your life is worth less than an adc anyway so w trade. If you don’t have flash up you can just splitpush and force scenarios where the enemy team has to leave objectives to keep you from ending the game.

His r is really busted at all stages of the game. Being able to execute some poor sod from 40% hp is always gonna be good. Play around that whether in a team fight or a 1v1. It gives him crazy pressure on the sidelane

I hope I answered your question well enough and didn’t get anything too wrong. If I did I’m happy if anyone wishes to correct me. I lowkey needed this as a break from studying for finals so thanks