For migrated shared mailbox, it AD account still hosted/managed on local AD, then sync to AAD.
This shared mailbox will changed to remote mailbox on Exchange on-premises, in this way, you will could manage this shared mailbox from Exchange on-premises.
You also could find those shared mailboxes with command below:
Get-RemoteMailbox | where{$_.RemoteRecipientType -eq "Migrated, SharedMailbox"}
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