Open the Outlook App
Click on Tools in the Ribbon and click on Accounts:
Click on your Microsoft 365 account and click on Advanced…
Click on Delegates tab and then click on the + under Open these additional Mailboxes: field
Enter the name of the shared mailbox, then click on it to select it. Then click Add.
Repeat the add process for all shared mailboxes you need access to. When complete, click OK
Quit and reopen Outlook. The shared mailbox will appear on the left column along with your own Inbox.