Logins created within the instance have by default no automatic access to any database within that instance. Each created user needs to be explicitly mapped to a database and assigned roles as appropriate.

The master database contains the structures of the databases on the instance and access to it does not imply access to other databases. It is really worth reading the security articles pertaining to fixed server roles and database level permissions to ensure to don't end up opening a security hole where you don't want one.