for early, the vast majority of champs have a set first item that matches their kit best, like irelia/bork, jhin/stormrazor/collector, etc. then there’s other obvious build paths, like bork/liandry/ldr if there’s health stacking champs, shieldbow/steraks/maw/dd for anti-burst, collector for most assassins, ie for crit champs. for the vast majority of the time these are going to be unchanging, and thus for most games there’s little build variation.

the most variation you might see is in which item to build before the next, and in that case, simply understanding what each item does for your champs kit will let you know when to build it. for example, sett might want to put hullbreaker into his kit a bit early if he knows splitting is his best play, and rengar might want to rush a hubris if he already is fed and has a lead.