Using ACLs to manage access to contacts
The ACLs in CiviCRM are only additive.  So assuming a user doesn't have the "View All Contacts" or "Edit All Contacts" permissions (which overrides all ACLs), the default is that a user can view zero contacts.

So to answer your second question: If you grant the user permission to access contacts in Group A, all contacts in Group A are accessible to the user - even if they're in Group B, which the user doesn't have access to.

This, in turn, should answer your first question - the ACL applies to the contacts in a group.