I found what works for me - here: https://www.elevenforum.com/t/how-can-i-find-my-windows-credentials-password.22854/
So... I'm trying to access a Windows-11 laptop from a Windows-10 laptop. The network is set up, but when I tried to access anything on the Windows-11 PC from the Windows-10 PC I was asked for credentials.
As many have asked: "What are these"?
The above link says they are (and this is unbelievable) as follows:
"The username you enter is what you see under C:\users on the machine where the files are shared. If you use a MS account this is the first 5 letters of your account name, then use MS account password.If you use a local account on the machine where the files are shared, use that username and PW instead."
So if your MS account is "anyoneabc@(something)" the username needed is "anyon", and the password is your MS account password.
Unfortunately I cannot remember the exact text on the window where the "Credentials" are requested, and I don't want to turn things off to find out (in case it all goes belly-up again). But I think when you get the error ("credentials not valid" or similar) there is an option "Use another account" (or similar) and take it from there.
It worked for me (again, after HOURS of research).