What is the route for a specific node?

I want to override the user.login route that points to the default login page.

I.e. I want that a user who presses the default "Log in" link for the site to be directed to a node that provides some help (where the user can click on a link and arrive at the standard log-in form).

Say that the path I want to redirect to is /node/42. How can I find out what route to return for a specific node?

I believe that the right place to do this in Drupal 9 is to override the getRouteName() function the in the LoginLogoutMenuLink class. So I'm trying to create something like this.

public function getRouteName() {
  if ($this->currentUser->isAuthenticated()) {
    return 'user.logout';
  }
  else {
    return '- What to put here? - ';
  }
}

I've searched a lot, and some people say that the route for nodes are entity.node.canonical. But that does not solve it for me. I think that the route for a specific node also need to have the nid in there somewhere.