Chad Callihan creates a new user:
I can’t remember where I heard the analogy, but think of a SQL Server Login as the key to a hotel. While a Login will get you in the hotel, you need a room-specific key (User) to access specific rooms (or databases) in that hotel.
When it comes to creating new logins and users, dbatools can help make it a more manageable process. This is especially helpful when you’re deploying the same login and/or user to multiple servers at a time.
That’s a nice analogy, and Chad follows it up with a pair of dbatools cmdlets you may find helpful.