Op is a bit confused, but here's a primer first:

SSH stands for Secure SHell and is a protocol to logon to a terminal shell via network.

You need to have an SSHd (or Secure SHell Daemon i.e a background service) running to accept and facilitate connections.

Systemd is a suite of services and tools that manage a Linux system, like a init system, service management, handing run levels, socket management, logging etc and gives the user tools like systemctl, journalctl, bootctl, basically anything ending with ctl is conventionally a systemd tool for users to manage their systems with.

Get it? Got it? Good.

systemd.autossh is an embedded ssh client in systemd that tries to help in reestablishing dropping connections. It does not actually start an SSHd (the actual service that facilitates connections) and is embedded for convenience to minimize frustrations with dropping connections.

You can read about it here.