Don’t understand how to set up ‘Custom Email Domain’

I’ve attempted setting up a Custom Email Domain to no avail. Instructions can be confusing for me. The ‘Example’; name@xxxxxx.com, for me, needs more clarification, as does the statement, “Add all of the email addresses that you currently use with this domain. You’ll be asked to verify them. You can create new email addresses after you’ve finished setting up your domain with iCloud Mail.” Whaat?!! I have tons of ‘Hide My Emails’ and some ‘privaterelay.appleid.com’ addresses too. Is Apple referring to my Primary or Custom Email? I’m getting many (? terminology) emails (junk? spam? or ??) I only view who sent them, never panicking by replying to fake purchases or calling phone numbers in them. I’ve been working on my iPad Pro (12.9-inch) (5th generation) No Cellular, 2021, 500+ GB Model.