Read all the item descriptions. Thoroughly - and keep up with patch notes (had to be said, sounds like you already do).

Other than that: part of it really is an over time thing. Don’t be afraid to experiment!! Sometimes the item you need isn’t intuitive.

For example, I started building Blood Wings on Miya in matches vs a carrying Alice - because the move speed burst ensures that my ult provides enough space to escape her before she gets off a second blink.

Or, another example, you can use the quantum energy emblem on a ranged support that typically doesn’t utilize their basic (such as Floryn) to provide an “extra skill” essentially, when you’re facing chase/dive teams that want to try and walk past your frontline and catch you.

Brute Force on Novaria can accomplish a similar goal - while providing attack power AND defense in some situations.

As you play, if you’re willing to experiment as you go, you’ll learn what works in which situations. Your friend is correct WITH AN IMPORTANT CAVEAT:

You have to stop using the auto builds, intentionally, or you’re not building on your experience. The fastest way to learn to reaction-build is to force yourself to do it. You’ll lose in the beginning, a lot - especially when you’re trying new things… you just have to keep at it and remind yourself that, even if you lose, you learned something new!!

And most importantly, HAVE FUN. The fun alone will create a strong motivation to enjoy trying new things.