unit (systemd unit)

Anything systemd manages: the unit is the object, and .service is only one of its eleven types, alongside .socket, .timer, .target, .mount, .slice, .scope, .device, .swap, .path and .automount. The suffix is part of the name, which is why treating nginx or ssh as the whole story costs you. systemctl status ssh works only because systemctl fills in .service; ssh.socket is a different unit with its own state. On Ubuntu 24.04 the consequence is concrete: systemctl is-enabled ssh.service ssh.socket answers disabled then enabled, because the socket holds port 22 and the first connection starts the service. Nothing is broken there — and systemctl restart ssh never touches what is actually listening.