What determines a player's rating? (S, A, B, etc.)

What detemines a player's rating? (S+, S-, A+, A-, etc.) I just got through playing support nautilius in a 53 minute game which is exhausting. My adc constantly positioned wrong leading to her going 19/13/27 KDA with only 228 cs. That's right only 228 cs in a 53 minute game!! I had 148 as support. I went 4/8/31 with the prementioned cs. I was in fact the only player in game with fewer than 11 deaths. I understand she was a part of a lot of kills but that was because I freaking soaked up the dmg like a sponge while her and the rest of the team mopped up. I did more dmg than the orianna who could one shot jhin. Yes, I am upset but I would like an explanation of really what determines a player's rating. There have been many times when I played awful and received an S or A rating, knowing that i really played like a B- or C. There is no clear understanding of what determines these ranks and I would like some clarification on this. Thank you for your time and consideration.