I couldn't recall ever noticing anyone use an icloud.com address as their primary personal email address, though searching through my inbox it has cropped up once or twice. Actually my sister did use one briefly in 2018.

And mac.com previous to that. Plus you can use your own domain name.

Are there any stats on how many people do that? It seems me the only way it could be true that Apple mail has far more users than outlook/live/hotmail is if the former tend to be associated with custom domains.

Apple mail the client has a lot more users than any Apple-offered mail.