Microsoft redesigned OneNote’s interface to be more consistent across devices - The Verge

See this Microsoft post for more details; tbd if/when macOS OneNote will support multiple open windows, as the full Windows OneNote app has done since 2003...

"Microsoft announced an update for OneNote today across Windows 10, Mac, iOS, Android, and the web, featuring an overhauled design meant to offer improved accessibility and a more consistent feel across the various platforms.

Navigation has been vastly simplified in the new app. Microsoft has moved all controls to be located on the left side of the screen in a series of nested menus, making it simpler to switch between sections, notebooks, and pages from a single place in the app. Additionally, the new design puts the actual information at the center, which Microsoft says will help students avoid distraction."

Microsoft redesigned OneNote’s interface to be more consistent across devices - The Verge