Musical Theory

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For the sake of not having a thread totally dedicated to one question I'd like to make this into a thread where people trying to learn music can ask questions for people that know musical theory

I'm trying to learn jazz piano but I'm very confused on how anything works
Most people will tell you that you just need to follow a simple chord progression but no matter how hard I try I can't use that logic to understand how something like
https://www.youtube.com/watch?v=D-4-Kg2qJt0
works musically
For starters a lot of the chords don't follow the Cmaj7 progression like how he's throwing a Cm7 in or a D7 when he should be playing a Dm7 and then some chords just don't fit that scale at all like the Ebm7
Can anyone tell me what is happening?