What are the informations accessible to the Hub and WPMU Dev?

So I've been playing around with the plugins and need to answer a couple of general security questions on how the hub is getting the information from each website.
1) Is there a documentation somewhere explaining how the pieces of information are retrieved
2) What is store into the WPMU Dev servers
3) What kind of access does the WPMU Dev team to each WordPress?

Is this information available ?
Regards,