Understanding MMR

Hi, I would like to better understand how the League ranking system works (specifically MMR). I know Riot doesnt like to go into details about it but I'm super curious. I won 8/10 placement games and was placed in Iron 1 with 60 LP, my friend won 5/10 placement games and was placed in bronze 4 with 25 LP. If its based on performance, I did get 2 A+s and 2 Ss throughout my placement games, so I thought I would end up in a higher rank. Is MMR based on previous seasons as well? Any help would be great, thanks.
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