"Overlay if", sliders "this layer", "underlying layer" what are they for?

Question

Adobe Photoshop

What does this layer option show and how does it work ?
A black and white marker can be split in two, or they can be changed together, why do you need to split them, and why is it unnecessary to do this in the option this layer ?
What do the values ​​20/60 - 180/220 show in this layer option ? What does the underlying layer
option show and how does it work ? The black and white markers can be split in two, or you can change them together, why you need to split them and why you don’t need to do this in the underlying layer option. does it give us?

What is this gradation from 0 to 255, why is it exactly like this and not, say, up to 365 or 1000, what does it show? Don't laugh, I really don't know).
Option overlay if has 4 values ​​what does it give us?
Grayscale
Red
Green
Blue
Thanks in advance for your answers.

Answer 1

These are settings for quite sophisticated manipulations of layer overlays and transparencies. No offense - but if you're just learning Photoshop - then RTFM, because you can't explain it in a nutshell, and a detailed answer will be a hefty sheet of text. If you have a specific problem - explain in what context