Hey guys can somebody explain this to me

Hey guys i dont understand a couple of things in this game. And those things are - how can i know what item for which champ i need to buy in random situations, i just go to probuilds and im always making the same build. Second how can i know if my team is stronger in late game or not or if im stronger than that and that champion at early or not, and the matchups and that. Can somebody explain, thanks.
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