Random email addresses are independent of and totally different from Custom email domains. I have been using "Hide My Email" since I first installed the Monterey beta and it works completely transparently and well. The way it works is when you use your apple email address to log onto a site (Login with AppleID) or give them your email address, instead of your actual email address the merchant gets an email address that is unique to that single web site and has no indiction of your actual email address. So instead of rumplestilskin@marypoppins.com all they see is . That merchant can send email to zzz2gmnskcs@privaterelay.appleid.com and it will be redirected to your actual mailbox at rumplestilskin@marypoppins.com. However, if spammer, or any other merchant should attempt to send an email to zzz2gmnskcs@privaterelay.appleid.com it will go directly into the bit-bucket at privaterelay.appleid.com because it did not come from the single specific merchant it was assigned to. As far as you, the user, are concerned, beyond the act you chose to logon user Apple, it is all magic taking place behind the keyboard and you never know the unique address the merchant sees.
As I now understand Custom Email domains, if you have a registered domain name, you can use it through iCloud without having to get a web hosting site and pay the fees associated with that. From experience those expenses can all too quickly run into triple digits preceding the decimal point. Even domain registration can get pricy if you aren't careful and avoid ,com, .net, or other common domains. Custom email domains are good for business recognition and vanity. Random email addresses provide a level of security and hiding
There is also a beta service called Private Relay. When that is turned on all of your internet traffic goes through a third party provider and the only IP address that can be seen is the IP address of the third party provider. Likewise an WhoIs or other internet query will only reveal the location and owner of the third party relay provider. You do have an option that the IP address will be in your general location or somewhere in the same country and Time Zone. I find general location to be better for e-commerce. Same country and Time Zone can sometimes give financial institutions every heartburn and cause credit card transactions to be refused.
Taken together Random Email addresses and Private Relay offer much, but certainly not all, of the security offered by Virtual Private Networks without the network speed penalty or cost.
