What does Azure Google provisioning do with existing accounts or accounts created in Google that are not matched?

I am trying to activate Azure provisioning on an existing production Google Apps installation. I am adding a single user in Azure and want to enable the provisioning. I am concerned about how this will affect the other accounts - the ones not related to the Azure provisioning.

So my questions are as follows:

1. Can the target SaaS platform e.g. Google have users that are unaffected by Azure AD? 
2. If I configure Azure provisioning with Google, what happens to existing Google (SaaS) accounts that do not have matching accounts in AD or accounts created in the Google space that are intended to be Google only? 
3. I assume the accounts are matched by user name? 
4. Do they remain independent?

Appreciate your time.

Brian.