can MacOS 11 serve Websites?

Yes.

As above mentions, Apache and MAMP and other web server options are available for or are integrated with macOS, yes.

If the websites are to be exposed to the Internet and to all the shenanigans that arise, the downside of this includes having to maintain and secure the web server and its contents, and the costs of potential mistakes and exploits to that server to other data on the potentially-breached server, to other potentially-insecure systems and servers on the same network. Running a server is slightly more involved than running a client. You'll probably also be learning about DNS setup, and about certificates for HTTPS, and about IP networking and port forwarding and potentially about public static IP addresses and DMZ / network partitioning, too.

Hosting services are cheap and widely available, as an alternative to running and securing your own web server.

Older Server can and did use TCP 443, yes. As I stopped using Server app when it ceased providing the network services of interest here and effectively became an MDM package at macOS 10.14, I don't have any experience with the current Server versions. Server does want to securely distribute some settings to clients, and I'd expect via an embedded web server. If you don't need an MDM or LDAP support or such, I'd remove Server app.

Your grep command is finding a process ID, and not a TCP port.

Your lsof is finding a web server running on TCP 80, and likely blocked (by Server? by the local config?) from TCP 443.

The following doesn't look that far off what I've used to activate Apache on older macOS:

https://neto.medium.com/apache2-configuration-for-macos-big-sur-serving-single-or-multiple-sites-2ae80a7ad406

If you're hacking around locally for development and experimentation, MAMP can work well.

I'd be very cautious about running any web server on an exposed or external or port-forwarded network, having cleaned up after various shenanigans.

How to run Apache on Big Sur

"Web serving is built into Big Sur with Apache app, it is installed ready to be fired up."

intandemdesign wrote:

Big Sur has an agent listening to port 443. Therefore, Apache cannot listen to port 443. If Apache cannot listen to port 443, it cannot receive requests to serve a page. HTTPS and SSL considerations if Apache does not have access to port 443.

Ah, interesting. Nothing here is listening on TCP 443 on the local Big Sur system, per lsof.

sudo lsof -nP -iTCP -sTCP:LISTEN

If the local system has something else on that port as happened with the old Server app environment, it was fairly routine to shut down or to redirect port-competing apps with Server.

As an ugly alternative, various firewalls can redirect incoming port access, too.