How does MMR work?

It doesn't. Good luck climbing if you are a non-carry with high MMR. You will get matched with low MMR carries who in turn will probably be matched against high MMR carries.

This oughta explain it pretty well: https://boosteria.org/guides/league-of-legends-mmr

what is The answer to the ultimate question of life, the universe and everything?

You see, it is this little hidden secret that is meant to deceive and confuse you as it "supposedly" is in existence in the game. No player has truly seen it and some claim it does not exist all together. Its job is to "balance" your matchmaking experience by matchmaking you with people in your skill range, which is not well defined. Observations show that sometimes you are on stacked or against stacked teams. No reason can be found for this anomaly or any indicative explanation given. most responses are in the form of seemingly random percentages with no background details alongside them.