Difference between child frame, popup frame and overlay

[Alessandro Bertulli]

Hi all!
Pretty much the title. I am exploring differences between packages like
Ivy vs Vertico vs Selectrum, or Corfu vs Company, and sometimes this is
taken into play. However, the documentation I found explains only child
frames and overlays. So, to be more precise:
- what is a popup frame?
- in what it differs from child frames/overlays?
- where are they used? I.e., can you give examples of packages that use
them?

Moreover, do you have any reason about when you should use one instead
of the other twos?

Thanks for your time and sorry if it's a too wide question.
Alessandro