Hover your mouse near the top of the screen to make the menu bar visible, or click the green button at the top left to leave full-screen mode. If you don't see the menu bar, you're probably in full-screen mode.
Most customization options are available on the Outlook menu bar and ribbon. Outlook for Microsoft 365 for Mac Outlook 2021 for Mac Outlook 2019 for Mac Outlook 2016 for Mac More.