This is a counting methods ( combination and permutation) problem.

I see the solution in a document on course hero but I don't understand it. I am trying to understand it

this is the problem:

Four friends want to divide a pie of pizza that has 8 identical slices (with no leftover).

How many ways are there to divide the 8 slices between them, with no restriction?

How many ways are there if each person gets at least one slice?