> Why you can’t toggle between different instances of the same app by default is beyond me.

I’m not entirely clear on what your requirement here is. Historically, Mac OS X/OS X/macOS has had a different app and windowing model compared to Linux. If you’re looking to switch between windows of a single app, you can use the keyboard shortcut Cmd + ~ (that’s Command and tilde). There are also trackpad gestures to see all windows of an application (swipe down with three or four fingers) and switch.