What I want on macos is that Finder can't have focus if there are zero finder windows on top and the desktop is not fully uncovered, maybe not even then. What already happened once for me is that I accidentally triggered undo thinking I had Firefox focused, because that was the visible window, and I failed to look at the menu bar, so instead I managed to undo a copy or move action in Finder, I'm not even sure what it was, because all you get is a plink sound and Finder doesn't help you figure out what you just did. It's a recipe for disaster, I imagine I could unknowingly screw things up big time.
I just moved to macOS for the first time, and my only way to adapt to its multi-tasking has been keeping exactly one window per open application, never zero or more than one. The fact that Finder can't be treated like that is a real pain. I will focus on it essentially randomly, and it will disrupt my intended interaction.
I don't get the reasoning behind the zero-window cmd-tab interaction, but if it is there I guess that there's a reason behind it?
On macOS you can have an app that is running without any windows open and you use the menu bar to invoke different commands in that app. This is why cmd+tab allows you to switch to an app that doesn't have any window open, essentially cmd+tab is an app switcher and not a window switcher. If you want a window switcher you can use AltTab an open source window switcher for macOS.
Why is there no finder specific setting that if it receives focus and has no window open it automatically creates the default one?
The current behavior is such a terrible user experience.
I don't know how a company as big as apple can leave everyday things in such a terrible state.
Consider that, with your proposal:
1) If you had a single Finder window open, then closed it, focus would get stolen by whatever other application happened to have a window open.
2) There would be no way to use the keyboard to interact with an item on the desktop without first closing or hiding every running application.
Truth be told, the desktop is already a kind of weird folder that is obstructed by all other windows, so I wouldn't really care about those types of inconsistencies. So for the first case, I wouldn't mind if closing the last finder window would jump to the next app in the tabbing order as if I quit it. The second case is a bit more tricky, but I think it's should be a matter of focusing only if I click on the desktop specifically and not in any other case, eg tabbing or closing other windows.