for learning

this is for learning cos dont understand what this means
How to Restore Files from Windows.old Folder in Windows 10

it says that this thing "C:\Windows.old"

comes from if a computer does either of these things -
custom install
upgrade
refresh

upgrade: a "upgrade" seems to only mean 1 os version to a completely different one
Upgrade to Windows 10

"refresh" seems to be one of those things that doesnt seem to work

custom install?
but what is "custom install"
what does it do and what does it mean
Custom Install Windows 10

a "clean install" means a new completely new install of same windows version? (dont know if that means a different windows version or no)
but what does "custom install" mean or do
do you know of any books or wikis about this stuff
that are clear


what in the world is it?

it seems to say that all user docs and files are what is in a "C:\Windows.old"
so it should be called "C:\olduserdocs" so that it could be clearer
it isnt explained what is a "user docs and files"
there no list of what things are "user docs and files"
like does chrome count as that or ?
this whole thing is so confusing