You need to imagine your webspace, as say, a tree. Your public_html folder is the trunk - inside your public_html folder are all the files and data for your primary domain name, that is your main domain name that you used to open your account with us.

If you wish to add more domain names to a single cPanel account, you need to create addon domains for them inside your cPanel.

These add on domains are like branches off the main trunk.

When an add on domain is created, this creates a folder INSIDE your public_html folder, by default this folder will be called the exact same name as the add on domain you just created e.g. if you created an add on domain called addondomain.com then a folder called addondomain.com will be created inside your public_html folder.

You should upload files and data for the website addondomain.com to the folder of the same name.

To access this folder, the easiest way to do so is login to your webspace via FTP using your primary domain details, i.e. yourprimarydomain.com (change yourprimarydomain.com for your own primary domain name), double click on the public_html folder, then double click on the folder called addondomain.com (or whatever your new add on domain is called), then upload your new addon website files and data to this folder.

You don't need different ftp details for each new add on domain, simply use your primary domain ftp details, then look for the folder inside your public_html folder of the add on domain name that you want to work on.