difference between a shared mailbox and user mailbox
Your understanding is correct that the main difference between a user mailbox and a shared mailbox is that the shared mailbox is associated with a disabled user. Shared mailbox is mainly used to allow logon access for multiple users. It is the same when multiple users use a user mailbox and a shared mailbox at the same time. A user mailbox can work as a shared mailbox, as long as you grant full mailbox access permission on the user mailbox to the other users. These users can log on the user mailbox.