When you set up an Addon domain, the system creates a folder with the same name as the Addon domain (unless you specified a different directory name in the “Document Root” field when setting up the Addon domain).
So, for example, if your add-on domain was ggexample.net, the folder would be: /ggexample.net
All files for the Addon domain ggexample.net would be uploaded to: /public_html/ggexample.net