I’m not sure what the confusion is, is it just that when you try to enter an email address the third-party service tries to fill in a domain automatically?
Specifically for myself, my email address is tied to my customerservant.com domain and it has been since 2005 and I’m not changing that. I do have a gmail address, the customerservant.com is hosted with Google Work Spaces, but I think it’s time to change that so I’m looking into other options.