Custom domain

Can you clarify what you are asking? Has has announced that iCloud will support custom e-mail domains in iOS 15 and Monterey. But this is strictly with e-mail. You are replying to an Apple Support article about storage. There will be no white-label iCloud storage available with custom domain names.

If you just want to leave Google and have your e-mail on your own system, you don't have to wait for Apple. There are a number of other e-mail providers that support custom domain names. I use Microsoft Office 365. It is quite inexpensive and Apple has better support for Exchange than for its own iCloud e-mail protocols. There are other e-mail providers that support custom domains, but Microsoft is the one I use.

For storage, you're on your own. I'm sure there are providers that make a white-label storage product. I'm not so sure I would trust any of them. You might be able to cobble something together on AWS. There might even be an AWS Marketplace solution for this too.

Are you using a custom domain with Google? If you are not you will need to register a new domain with a registrar like GoDaddy (BTW Apple and GoDaddy have the setup pretty well automated). One thing to understand is the this will be set up as an alias to your AppleID making it essentially a unified inbox. You will not be able to use this email address as you AppleID (I have verified this.)

If you are using a personal domain with google already you can make the changes in you iCloud account and then make them in DNS management with your domain registrar if Apple hasn't already integrated with them. It's only a few DNS records to either add or modify.

Though I haven't been able to completely verify at this moment, calendar integration on the Mac isn't working very clean either. I had to add the meeting from the said attachment that was coming from Office 365 to the calendar into iCloud. I am still testing at this time but those are the things I have found so far.