Pro Sites: Where is the core Site hosted

I'm interested in the pro sites plugin. I watched all your videos. But I still have one question:

After a potential clients register a site over my network, it will automatically install a WordPress site, right? My question is, where is the core site hosted, and can I customise it? I want to modify the basic wordpress site and when a new client register a site over my network, it will install the modifyed wordpress installation.

Thank you in advance and sorry for my english.

Mike