Soft-deleted mailboxes are generally purged after 30 days. If the mailbox is on any type of hold at the moment of deletion, it will be converted to "inactive" mailbox instead. You can list those via Get-Mailbox -InactiveMailboxOnly.
Inactive mailboxes will be preserved for the duration of the hold, this includes litigation hold, In-place hold, retention policies, etc. They do not consume licenses. More details here:
https://docs.microsoft.com/en-us/microsoft-365/compliance/inactive-mailboxes-in-office-365?view=o365-worldwide
