Just need some honest feedback and help to improve myself

Ive played since S2, and every single patch i feel like this game just keeps going downhill. each patch makes the game more complicated, only making league of legends playable by real pros, players that have no life that can sit down in a chair for 20+ hours a week and grind their skills.

Point is, Ive noticed several items that CLEARLY DO NOT WORK. Such as BOTRK, stating "UNIQUE: Basic attacks deal 8% of the target's current health (min 15) bonus on-hit physical damage (max 60 against minions and monsters)." This passive applies to all targets, including minions, champions, and even dragon/baron. SO WHY when I play any game(weather its bots, or even the practive tool to prove it or a pvp on Sum Rift), why does the damage not apply? EVERY auto attack would technically do less and less damage(assuming the enemy had no healing whatsoever). Lets say i have a perfect 100 AD, Enemy had 100 armor giving a perfect 50% damage reduction against physical attacks. The enemy has 100% hp starting at 1,000. So my auto would do AD/Reduction + BOTRK passive = 100/2 + 8% of his health which in this case would be 80 total damage. therfore that single auto attack would do 130 total damage. Then the NEXT auto attack would do less bc the enmy has 130 less health. Making him have 870, so same scenario, 50 dmg plus 8%(which is now only 69.6) which would make my total damage for my 2nd auto attack do 119-120 damage. BUT IT DOESNT WORK. Instead... I only do 50 damage with the first auto. then 50 with the second.. and 50 with the 3rd. use the item, then come back and explain how everything works. please i want to udnerstand to become better

I dont understand how damage works, Runes are much to confusing, and you dont know what runes the enemy has(not specifics anyway) so how do I build against them? this item along with Ninja tabi... i have tested it, Each auto attack i receive from the enemy is NOT reduced by 12% instead is is reduced a RANDOM amount. build yourself 20 armor, test damage, and after, retest damage with the 20 armor(and passive)=Tabi and it is NOT taken 12% off. instead it will shave off something random like 5% or 14% then the next time you test it it will be 2%... sooo anyway you see my dilima. Almost every item with a unique passive. Clearly is broken/bugged.... That or other players are hacking, but Riot would never admit to either of those options. so here i am left with no answer form Riot and just trying to become better bc i am highly frustrated. Thank you.

Just need the biggest answer you can provide me on How League of Legends works! please help...Thanks guyz.