As you can see, there are two different methods to sync an iCloud calendar with Outlook. In the section named “Open these additional mailboxes: “, click the '+' button.
In Calendar view, … repeat step above to see that it has synced Outlook for Mac always displays calendars in overlay mode when it shows four or more at a time. to set up and manage your email and calendars using Outlook for Mac in Microsoft 365 (formerly Office 365).