Diff btw Command Substitution And Pipeli

Newbie question:
I’m puzzled at this pt of learning: What’s the difference btw the usage of comm. substituion (`) and pipelines (|) ?
I mean HOW do you determine which one to use
to feed Inputs ? They are NOT interchangeale under any circumstanceat all, are they ?
Pls be as specific as u can..I’m pretty new to this. Thanks