iCloud+ Custom Email Domains: what, exactly, does this do?

I have existing mail hosting on a domain I own. I have personal and family email accounts setup there as well as some systems management addresses (webmaster@, etc.) I also use non-Apple devices and email client apps to get at these accounts via IMAP/POP3/SMTP in addition to having them setup in Apple MacOS and i*OS Internet Accounts. I don't use the Mail feature of iCloud.com, the website. I'm trying to figure out what, if anything, the iCloud+ Custom Email Domains feature gets me/what, exactly, the new iCloud+ Custom Email Domains feature does. Does it:

a) capture the DNS MX records so all SMTP mail gets delivered to iCloud Mail--making iCloud your @mydomain mail host for SMTP and IMAP/PoP3 setup on all non iCloud mail apps (and, implicitly, suggesting that all email for those domains is going to get sent to iCloud hosting, not just the addresses registered during the iCloud.com Custom Email Domain setup)? or,

b) configure your @icloud.com email to also fetch all your mail, via PoP3 to existing hosting, and send via SMTP, via existing hosting (similar to what Gmail does ("Send mail as"/"Check mail from") and what Outlook.Com used to allow)? or,

c) does it merely enable iCloud Mail to use existing mail hosting/storage via IMAP/SMTP so that you can see/send your custom domain email in iCloud.com mail similar to adding an Internet Account for mail in MacOS Mail, i*OS Mail?

Thanks for any clarification. I'd just go ahead and complete setting it up, but there are lots of other non-Apple/non-iCloud things that would get broken if it does a) or b)...