how do i know when i build what ?

Hey there, So i am playing LoL now since a lot of years (i am not a LoL fan, which makes me to go inactive very fast for a long time always cuz i am not a fan of moba games) just for fun. Till recently i never cared about building / items. I was playing always garen till yi came out and then i switched to yi and never chose any other champ.

I checked online about how i should build him and i tried these guides and i won most games. But then i had some other games, where i did enough damage but died way to fast.

This month i started playing again and i decided to play more intensiv than ever. So i checked youtube for Item and stat guides etc. and i understand now finally the basics.

But still, i dont know WHEN i should buy WHAT.

So after learning the basics, i started a casual game and i had darius at top lane (i never go jungle. I hate it.). I started with to finish once i got enough money.

I realized that darius makes some good damage. So what i did was, i left clicked on darius and looked how he built. He started with more HP. So i avoided attacking him cuz he needs less hits than me to kill me at the beginning of the match. And according his stats, he had about 70AD and i have about 46 defense / armor.

Once i got enough money, i got (i got that item straight after ) and then i was able to kill him finally cuz he needed more hits to kill me. Then i focused back on finishing . After i finished blade of the ruined king, i realize that darius makes now way more damage and is able to kill me with again less hits than me. So i upgrade my to and then darius cant kill me anymore and i can keep feed my self from killing him when he tries to get more minions.

I had many darius champs at top lane and this really worked well and because of me, my team won (i even had a match where 2 of my mates left and it was 3vs5 and still i did a lot of pentas etc. without dying more than 2 times and push the enemy insane and finish the game).

But also, i had some games, where the enemy from mid lane had 1K AP and i had no chance to beat him. Even if i would be 100% hp and he 50%, i couldn't make it since i built on blade of the ruined king and some armor and not magical resistence.

What could i do, change, etc. ? any help would be really appericated since i realized how much fun the game can be if you understand items and how to build O_o.