> All macOS is is just a different window manager

This dramatically undersells what MacOS is and was. It was way beyond just a window manager.

From its inception in the 1980s it included a set of APIs that allowed developers to build sophisticated (and consistent) GUI applications with comparatively little effort. eg Quark Xpress, Illustrator, Photoshop, Excel, Word

By the end of the classic Mac era in the late 1990s that API set had grown to include a ton of stuff. QuickTime, ColorSync, TrueType, AppleShare, sophisticated printer support, multiple display support, etc