How exactly does the ranking system work?

Is it set up where each champion requires a set of unique specifications to achieve an S rank, like assassins require a specific amount of kills and low deaths or supports only need to a hit a assist/death ratio that leaves them eligible for the S ranking. I know minion kills are a large part of it as well but does it also take tower kills or epic monster participation into account?

I ask because i recently finished a game in aram where i went 17/5/36 with a xerath, with 123 minion kills, and only received an A. I sorta get that since players tend to get 20+ kills with a good champion in aram so maybe i didn't reach the kill count for an S, but then I remembered playing a summoners rift game with Ekko where i went 21/5/19 with 151 minion kills and only got an A.

This ekko game left me questioning, since two other players received S rankings even though i did better then them in almost every aspect of game play. One of them had a slightly higher minion score and assist number, but on all other accounts i was technically better.

So i was just wondering what other factors of gameplay are taken into consideration in the ranking system and what is needed to receive an S ranking. ty