You cannot, although it's a common ask. Go vote for it here: https://feedbackportal.microsoft.com/feedback/forum/89a8afa3-2e1c-ec11-b6e7-0022481f8472
"all inboxes" view in outlook 365?
Peter Nauenberg
1
Reputation point
How can I create a view in outlook 365 that shows the content of both an individual AND shared mailboxes hosted in M365 exchange? This feature is available by default in the IOS native mail program.