Using more chords and changing them - Guiltlessly

Ok, so we know that the standard major scale gives us a Major, Minor, minor, major , major minor and a half diminished - e.g. C,Dm Em, F, G, Am Bdim etc. Well, evidently (and historically) changing the minor chords to 7ths and even sometimes changing a 4 chord to a minor also works. I'm not clever enough to understand why I just know it does so I'm sharing some examples. I do these videos to help an share, but this time, if there are folks out there that can explain this to me, I'd welcome their comments!