Port 443 is the default HTTPS port. HTTP/1.1 and HTTP/2 run over TCP with TLS, HTTP/3 (QUIC) over UDP 443. If you enable HTTP/3, open UDP 443 too. Many services also tunnel through it, e.g. TURN for Nextcloud Talk or VPNs, because 443 gets through almost everywhere.
Public for web servers. Important: serve the full chain (fullchain.pem), TLS 1.2 and 1.3 only, HSTS.
Common errors: unable to get local issuer certificate from a missing intermediate, the wrong certificate due to missing SNI mapping, and expired certificates when automatic renewal fails silently.
Check
openssl s_client -connect example.com:443 -servername example.com