Apple doesn’t directly host domains. Apple partners with Cloudflare for domain registration. You can move your domain to Cloudflare, though. Don’t cancel your domain first, or you may not be able to get it back. You’ll initiate the transfer at Cloudflare, who will then provide you with all of the instructions for what to do next.
Once you have your domain with Cloudflare, you’ll want to update all of your DNS records (including MX records for your email services). It’s also worth knowing that you don’t have to have your domain registered with Cloudflare in order to use the custom domain email.
Cloudflare has some info here: https://www.cloudflare.com/learning/dns/what-is-cloudflare-registrar/
Apple has some good info here if you’re planning to use custom domain email: Set up an existing domain with iCloud Mail - Apple Support