If you use Exchange (either on-premises or online), you are probably already using the Online Archive (also known as In-Place Archive) which is a ‘separated/hidden’ mailbox storage to help reduce the size of your mailbox (see Manage email storage with online archive mailboxes – Microsoft Support).
Well, while access to the Online Archive has been available on Outlook clients on Windows, MacOS or using web access, this was not available from Outlook Mobile.
Good news, as now with the latest Outlook Mobile update, access to Online Archive is now also possible from iOS or Android devices.
Off course, this requires that you got Online Archive functionality enabled for your mailbox (see Enable archive mailboxes for Microsoft 365 | Microsoft Learn or Manage In-Place Archives in Exchange Server | Microsoft Learn)