If the "what's happening" link is missing from a server you're looking at, or if it's filled with posts from other servers, contact the server admin.
Explain (politely!) you want to take a peek at the public timeline and send them this link (https://mstdn.social/@feditips/108214965120963080).
The admin's contact details are at the "Learn more" link on the front page of their server.
@feditips I'm new here! I see some users with accounts on "mastodon.social", but this server doesn't seem to exist, though "mstdn.social" does. What's going on there?
Mastodon.social definitely does exist, it's at https://mastodon.social. It was the first Mastodon server, it's owned by the lead developer of Mastodon. It has sign-ups closed though as they're trying to encourage people onto smaller servers. It's bad for the network if there are too many people on one server.
https://mstdn.social is a totally different server with a different owner.
They're just two different servers that happen to have similar names.
@feditips
how strange that you need to insist that people should be polite and respectful
that should be evident, but it seems not, unfortunately