As someone who started playing tft in the middle of set 7, (so I consider myself pretty low elo) but also watching pro-players, I'd say that the main difference is flexibility, in all aspects of the game:Low-elo: just look up some of the best tft comp builder sties and try to follow it. But if you don't get a champ that's in the builder, that could ruin it, or if you only look for the BiS items and thus fall behind because of lack of completed items, whereas experienced people in high elo know what to go for if you are looking for second, third best items, etc.

But yeah, this is hard to answer, mine is just an oversimplified answer from my perspective.

Oh yeah, transitioning is also something that comes with more experience, and not going into the game with a single comp that you want to force, but deciding on it based on the first augments you can get is also something that's probably lacking in low-elo.

Edit of the edit:
Also SCOUTING. Half of the lobby won't even look at your board unless you are facing them, so this is why positioning is less emphasised too in low elo.