Balance questions

I've only started playing (console) but I've always been fairly aware of the game. I was wondering what do they change balance patch wise. I see characters like viper,kay/o,omen and KJ played often in pro play even on videos around a year ago those three are mostly dominant. I've also heard in ranked gameplay someone like kay/o has a negative win rate. Yet I still haven't seen any changes made to certain characters (I'm not advertising a kay/o buff) balancing doesn't seem to happen often.

Just a rant a diverted off main topic. I'm just wondering how they make balance choices.