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.