GTM health monitior vs. service port

I am trying to understand the differences and relationships between GTM "health monitors" and "service ports". When I add a new "Server" in GTM, I have to indicate the "Service Port" (ie: HTTP, HTTPS, 389, etc). But this port seems to have no relation to whatever health monitor I assign to either the server or the pool. I have a few pools with servers with no health monitors configured on either the pool or servers, but things work correctly. I recently had one pool flap but there was nothing logged in the GTM log about it. Questions:

1) If there is no health monitor assigned to a pool or a server, how is the health checked, how often is it checked?

2) If a GTM Service Port is set to, for example, port 80, what specifically is being checked? Just a TCP connection on 80? Or is the GTM issuing an HTTP GET and looking for anything to be returned?

2) If, for example, a server has a Service Port of 80, and a health monitor probing 8080, are both elements required to succeed for the thing to be green?

3) If the Service Port of a server is not reachable, is this logged in the GTM log?