The Cmd-Tab to switch between apps is good because it’s Least Recently Used (LRU) ordered, and the there is a visual to show what you are choosing.
Once you’ve done Cmd-Tab to choose an app then you need to do Cmd-` to choose the document. But this is usually not LRU and not visual.
In Firefox I can configure LRU and visual tab switching, but not windows switching. And Finder or IntelliJ I can cycle through windows but not in LRU order.
I have a good mental model of my apps, windows, documents and tabs, but my mental model is LRU based, not cycle based.
LRU is good but before that becomes the problem, there is the problem of requiring 2 steps when all the user is thinking about is "back to the previous document", or "let me select one of the documents". No one gives a shit about apps
Don’t get me started on Cmd-Tab will full screen and Spaces!!
The user is supposed to build a mental model of applications and windows in their head and think about that carefully when switch.
Apple's thought very hard very different, and deliberately build their UI that way to keep people's minds sharp. You're welcome.