HTTP Service URL

I notice that in docs and examples the way the URL field is populated varies. Some places include a trailing slash, while others do not. Sometimes an IP address is used and sometimes a hostname is used. Also, why have a http or https?

For example, I might have something like this that would be used with an Apache server hosting the site mydomain.tld:

      rule = "Host(`mydomain.tld`, `www.mydomain.tld`)"
      service = "mydomain"

        url = "http://mydomain.tld"

I'm curious as to what effect those variations have? Does it change what is ultimately passed to the Apache server? Does using a hostname or IP address it change the host header? What is the purpose of having the trailing slash or omitting it?

Since the site is accessible with 'www' and without, does this mean (according to my config above) when someone is accessing the site with 'www', it's actually being passed without the www to Apache?

Finally, if using a hostname instead of an IP address, how long will the DNS lookup result be cached?

Could I just use the PROXY v2 protocol between Traefik and Apache instead?