All you need to do is create an empty text file called .htaccess in the folder containing your desired default web page. Then, you have to add the following line to the .htaccess file:

DirectoryIndex something.xyz

where something.xyz is the file name of your desired default web page.

This will be the web page that loads up by default when someone accesses that folder with their web browser.
Each folder (including the /public_html folder) can have its own .htaccess file. Since each subdomain has its own folder, this means that the default web page for your domain and each of your subdomains can be easily specified.