When auto-expanding archiving is enabled for a user's mailbox archive or a shared mailbox archive, Microsoft Purview periodically checks the size of the archive mailbox. When the archive mailbox gets close to its storage limit, additional storage space is automatically created and this process continues until the mailbox archive reaches 1.5 TB
https://learn.microsoft.com/en-us/purview/autoexpanding-archiving
https://learn.microsoft.com/en-us/purview/enable-autoexpanding-archiving
https://learn.microsoft.com/en-us/powershell/exchange/connect-to-exchange-online-powershell?view=exchange-ps
1) Connect-ExchangeOnline -Credential $UserCredential -ShowProgress $true
2) Enable-Mailbox ***@*****.com -AutoExpandingArchive