Question

With the base install of nginx, your sites-available folder has just one file: default

how does the sites-available folder work and how would I use it to host multiple (separate) websites?

Answer 1

Just to add another approach, you can use a separate file for each virtual domain or site you’re hosting.
You can use a copy of default as a starting point for each one and customize for each site.
Then create symlinks in sites-enabled. In this way you can take sites up and down just by adding or removing a symlink and issuing a service nginx reload.

You can get creative and use this method to redirect sites to a maintenance mode page while you are doing site maintenance.

So the structure looks like this:

/sites-available/ (you can use obvious file names like this)
| 
|-> a.mysite.com
|-> b.mysite.com
|-> someOtherSite.com

/sites-enabled/ (these are just symlinks to the real files in /sites-available)
| 
|-> a.mysite.com
|-> b.mysite.com

Notice that since there are only the first two entries are the only symlinked items in sites-enabled, the third entry, someOtherSite.com is therefore offline.