After installing my master site, I created a slave site (following the tutorials and video). This is a fresh slave site.

I also ran the hello.php to test the symbolic links and everything was shown correctly with no problem.

I checked my files from the File Manager in cpanel and I find that the files in the slave and master are identical. Is this correct? All the administrator, component, modules programs are stored in the slave directory as well. I thought it should be just a symbolic link or am I wrong?

Does this mean that if Joomla is upgraded in future, all the files will be automatically upgraded when I upgrade the master?

I suppose that you have deployed the slave site into a specific directory.
The problem with the cPanel file manager is that you don't see when this is a symbolic link or a physical directory.
So this is normal that you think to see a replication of the files and folders.

I suggest that you use another tools to correctly see if the files are symbolic links or physical one.
This is the reason why in our video, we have use the FTP with "totoal commander" that display when this is a link or a physical directory or file.

The objective of the symbolic link is to show exactly the same files and folders than your master website and avoid using disk space as you could consider them as synonyms of your master joomla installation.
So this is absolutely normal that you see exactly the same directory structure and files

As explained in the concept presented in the tutorial video 0, the objective is to share the same files and folders.

Yes, I have deployed the slave site into a specific directory. I will try to use another tool to see if the links are symbolic links. Thanks for your quick response.