I have noticed that when ever I start nginx with ubuntu command nginx and I do systemctl status nginx. It shows that systemctl is disabled. More over if I first start nginx with command systemctl start nginx and I try to start nginx with command nginx, it check the availbility of the ports and then says
nginx: [emerg] still could not bind().
So I thought there must be a difference in their purpose. When I start nginx with command nginx the only way to stop nginx is by the means of force using killlall nginx or kill -9 (process id) or by clearing the port. So I am pretty sure there is some difference in them.
The difference between the examples you have provied is how the processes are being started.
Running the command nginx will start the application and wait for your user action to stop it.
The systemctl or service commands are nearly the same thing and running service nginx start or systemctl start nginx will start a service in the background running the Nginx daemon.
You can also use this to do a service nginx restart or systemctl restart nginx to restart the service, or even a service nginx reload / systemctl reload nginx to reload the configuration without completely stopping the Nginx server.
The reason why you can't do both nginx and systemctl start nginx is due to the nginx configuration already listening port 80, and you can't listen on the same port on a single IP address at the same time.
You can also force the nginx service to start on boot by running systemctl enable nginx which will be why your systemctl status nginx returns 'disabled'.
Hope this makes sense.