i like to know how the mmr works

i am no longer playing lol .... well it's like 2 months or 1.5 i don't know ..... and every time i rethink of playing this game .... i see the new champs coming up and the items that i used changing to be bad for me ..... but i know that is how the game work .... still ... there is somthing in this game i love maybe my main .... maybe ... i know that i won vs every zyra player so far even if my adc is bad ....even in rank when i don't play her and they pick her i know how to win vs her ... but .... every time i think HOW WILL I WIN AS A SUPP MAIN ...... THAT IS making my mind stop working .....
now that i made my mind up i like to play the game once more but first i like to know more ... how this mmr work ..... so i can play on it .... coz i jumped to plat 3 in 1 day or 2 i think then boom plat 5 now that i look at my acc i see it's gold 1 coz i am not playing anymore .... well if there is any adc here like to work with me in this .... pls tell me .... and it's funny coz i was looking for an adc since 2015 and still can't find one ... [slayer-jinx-unamused]