Does anyone here actually memorize what every single items in this game does?

I read and remember.

I don't bother remembering exact numbers but I do try to be aware of what stats items give at all times as well as their passives. Just comes from reading them a lot back when I started caring about item builds and then keeping up with patch notes since then

Yes, and I also memorize the cost relative to the pieces I already have. It's just practice.

Play a couple of years maybe

Focus on the general and not the specifics. You will get the specifics down later.

You get up to date at some point by playing the game a lot and tabbing to see item builds on habit, then you just read patch notes regularly and play some more. It's not hard, it just takes time.

You mean there are people who don't know what every single item does? If you're not going to bother with the very basics of the game, such as knowing what items or champions do, why bother play it? Clearly it's not the right type of game for you if you can't be bothered with the absolute basics.

As for how I memorized them... it's a very tiny amount of things to remember.

I do, but that is because I associate them with something. Like DMP is the cape-granting armor item, randuins is the "come at me yasuo i got 6 randuins" armor item, void staff is the pinnacle of penetration, boots of mobi is conditional super speed, but boots of swiftness is more guaranteed speed, things like that.

Practice, style of play, top champions, and effectiveness in games.

I read everything. I remember unique passives (that aren't like Serrated Dirk's 10 Armor penetration, real uniques like Infinity Edge), the stats it gives (AD, AP, CDR, MR, AR, HP, etc) and generally the exact values aside from HP. I also memorize build paths and the general area the cost falls into (ex. tri force and IE are 3800ish).

I read the description, then i remember what it does.