I'm not familiar with how the integration works; but if you enter your keys and activate it, I'm assuming that in the client interface, a "Website Builder" or similar option will appear, which when clicked by the customer, will take them to the site.pro integration (which I believe connects through FTP to upload the resulting files to the servers).
These questions are better answered by taking a look at the source code, or from hands-on experience (trying out something yourself, either in a local or your production environment); unfortunately, if you're not very familiar with PHP, you may need to start learning it; GenerateArea is in a transition period currently where development is focused on a new client area, so support is limited. Even if it wasn't though, it's a good idea to be able to understand the software that's managing your clients, and some familiarization with PHP is necessary if you're managing everything on your reseller.
Why not try and figure it out yourself? You could have found that out in max 1 hour (for normal people). But now, it has been 8 hours(at the time of posting this), and nobody has given you an answer? Also, asking questions without doing your own research is extremely annoying, so please, do your own research, test it out, before asking questions that are so simple. You can get the answer must quicker if you do research and test it on your own. Asking on this forum makes that you have to wait a whole day to get an answer that you could have possibly get within 15 minutes, and answers on this forum may not be 100% correct.