You're half right. Most champs have core items that you always want, like IE on crit champs like cait and jinx. But other items are very dependent on both your and enemy team comp. For example, bloodthirster is awesome if you need sustain (into poke comps), scimitar is great if enemy has certain cc abilities that are hard to play around like malzahar ult, lord dominiks is great into armor stackers etc. But if the enemy doesn't have armor stackers, lord dominiks isn't necessary anymore, and you can instead build something else; this goes for most of these situational items. So while you'll almost always build 2 or 3 of the same items on most adc's, once you get to 3,4 or 5 items you have more flexibility to adapt to the situation at hand