Welcome to the community forum. Just a quick one, are you familiar with how the platform works when it is within our servers? If so then basically various handling procedures would be the same. On the specific areas asked:
Yes, the data is stored within your servers and we have absolutely no interface to your data.
You should be able to access the list of your users on the admin side of your installation. This should be somewhere similar to https://YOURSERVER/admin/auth/user/
Remember this will provide you with a list of your users but not the ability to add since this is done directly on the platform by the user.