How do remote agent apps like TeamViewer work?

I am building an app which will require an agent app and a server app. The server app will need to communicate with the agent app, over the Internet.

How do apps like TeamViewer work without the users needing to modify their firewall? Does TeamViewer connect to the remote agent via Http? Or is it some other protocol?

My app will work in a similar way to TeamViewer (it is not a remote access app however, the principle of having an agent that is accessible by a host is the same).