Copyright claims on videos - conditions?

[LandyVlad]

Does anyone know the answers to these questions?

Is there any difference between whether copyright claims are enforced on private vs unlisted vs public videos?

How are there so many unofficial music video channels that seem to go along no problem, but using a brief section of movie or video clip or a piece of music in a YT video on another subject is often hit really quickly with a claim or strike?