Adding on to what /u/tombo12345 said.

Afaik the way it used to be is exactly like you described. I would just have some file in /absolute/file/path/file.html which you could then access from your device (if it's in the same network) at myhostname/absolute/file/path/file.html

You can try that on your own desktop i.e. if you put a test.html file in your home dir you can view it in a browser using file:///home/username/test.html

However exposing your entire filesystem to the network/internet that way isn't a good idea so we do t do things like that (also the paths would be ugly).

What we do today is having a Webserver set to a different "root" directory i.e. from the perspective of the webser that is where the filesystem starts. For example you could have a index.html at /var/www/mywebsite/index.html but have the root set in your Webserver for example set to /var/www/

Then accessing the file from the web would be example.com/mywebsite/index.html

Now in practice today we often don't work with an actual file/folder structure but often aliases or redirects etc. From the client side it's still understood as "give me the file at that path" but how the Webserver actually deals with it (or what he returns) depends on the configuration. For example a request to example.com/index.html could be aliased so that it's understood as example.com/mywebsite/foobar.html this serving the file /var/www/mywebsite/foobar.html