A shared mailbox is the one, from which many users can  read and send email s. Shared mailboxes can also be used to share a common calendar, allowing many users to make a  schedule and  see the details of work shifts etc.

Migrate mailboxes to shared mailboxes using EdbMails :

EdbMails Office 365 migration tool give the detailed steps to create shared mailboxes in Office 365 and to migrate your mailboxes to shared mailboxes.

To know more  Office 365 Tenant to Tenant Migration | office 365 migration steps | office 365 migration Guide