What is the OP.GG MMR? and how is it different from the old ranking elo system?

I know OP.GG isnt accurate in any way, but im just curious how it works, its really interesting to learn about how the older ranked system used to work.

What got me curious was how today when I played 3 ranked games I lost 2 and won 1. technically i lost just 1 game, and at the end of that when I went to OP.GG i saw that my MMR points had dropped by 79! 79 points for ONE LOST GAME? iv never seen such a huge drop even relative to when i was on a major losing streak.

Can anyone shed some light on this?

Thanks in advance guys and gals, this is purely for personal use and curiosity, I do not promote any websites.