Two separate things.
WHAT CREATING A SUBDOMAIN IN CPANEL DOES
Creates the directory, configures the web server to serve it, and adds a DNS record to the cPanel zone.
IF YOUR NAMESERVERS ARE OURS
That is everything. The subdomain resolves and serves.
IF YOUR NAMESERVERS ARE ELSEWHERE
The cPanel zone entry is ignored. You must add an A record for the subdomain at your external DNS provider.
Until you do, the subdomain does not resolve, though cPanel shows it as created.
That is a frequent source of confusion.
WHAT TO ADD EXTERNALLY
An A record for the subdomain, pointing at your account's server address.
SSL
Run AutoSSL after the subdomain resolves, ticking the new name.
A subdomain without certificate coverage produces a browser warning.
REMOVING A SUBDOMAIN
Remove it in cPanel, and remove the DNS record externally if applicable.
An orphaned DNS record pointing at a subdomain that no longer exists produces confusing results.
CHECKING
Look up the subdomain. It should return your server's address.
