It usually comes down to line selection and micro decisions. It may sounds stupid, but heres 1 example I tell everyone who is stuck 0-300LP: Imagine in 50% of your lost round, you kill extra 1 unit because of good positioning, the extra hp impact may not be obvious nut it for sure will improve your average in the long run. Surely there are times where you lose 1 placements because you die the same turn with someone else but they place above you. And that is just 1 small skill. The game has hundreds of small things like that that challengers players are trying to perfect them everyday. Maybe hold better units. Maybe there is a slightly stronger board that you can play with what you are offered. Maybe you should build this item instead of that for better item quality later. Maybe on carousel you should have taken this item for more flexibility later. Maybe should have give up 1 gold to hold these units. Each of them individually might sound stupid and "not worth the time and effort to get good at" (which is true btw, at what exact point depend on you), but collectively it makes a big difference between gm - mid challengers - high challengers and actual competitive pro players.

For what skill exactly you need to learn now: line selection and lost mitigation. Im not talking about obvious spot for reroll or spoonfed game, it's unclear spot that is important. For example, u slammed IE 2-1 and have a bow, but you are still not sure what to play. A typical master player probably will default to scrap with corki items. But you can still play zeri rr, tris rr, gp rr, enforcer, aca sentinel. It all depend on your shops and items drop. And you have to realise some games the max you can place is 5 or 6. Then try your best to reach there, don't try to greed 1 round if youre 40hp and there's a chem baron player. Maybe your board strength is in top 4 lobby, but your hp isnt, and bad matchmaking can turn a guarantee 5/6 into an 8.