Adding files to the directory tree doesn't affect the nginx server, but changing a configuration file does. You'll need to tell nginx to read its configuration files again. Use

sudo service nginx reload

You can find all the commands available by using

sudo service nginx help

Firewall rules

To create a firewall rule, you'll have to use the GCP console. See the attached image for where this is and what the settings should be. You want to enable a small range of ports, like 7070-7075, since you will be hosting several different web sites. Make sure that the Target is http-server. Note that the default rule is already there.

Once the firewall is opened, you should be able to get to your new website with a URL like http://1.2.3.4:7070, using your IP address and site port number. Youll know its the correct site, because you have already modified the index.html file to show this is second site.

Take a screen shot showing your browser window with the URL, along with the modified front page.

Create a third site

Now repeat this all one more time, just so you get to practice the steps. The good news is that you don't have to create another firewall rule, as long as you use a port number in the right range.

Take a screen shot showing your browser window with the URL, along with the modified front page.

When you are finished, you will be able to access 3 web sites, each using a different port number on your server.