It's one of the option during the first experience configuration of Windows 10. If you had another browser installed, one of the toggle is : do you want to use Edge as default browser. Hopefully, people that feel that strongly about their browser choice don't just use "Express settings" when installing a whole operating system.

The OS update installs a new browser, which then sets itself as default.

Windows isn't installed automatically.

Let me lay out the analogy more explicitly. I install Firefox, and specifically choose for it not to be my default. I later download an updated version and choose default settings in the installation. Firefox then sets itself as the default browser; it overwrites my previous choice in favor of the default.

I install Windows. I specifically don't set explorer as the default. I install an updated version of Windows and choose default settings. Windows overrides my previous choice.

Right, neither should be doing that in the case of an update. Firefox when updated in the normal manner doesn't make itself the primary browser. Windows 10 is behaving badly.

Firefox will only change things if you specifically get a Firefox installer, where it thinking that's your intent is understandable.