Since I own the Windows software - it appears to me - extremely odd that I can't decide what programs I am allowed to use on my PC
Actually, you don't own the Windows software. You pay for the right to use it. I'm not aware of any commercial software which gives users ownership rights. Microsoft owns Windows.
Microsoft needs to keep Microsoft Edge in Windows because the back end (the technical part, without the interfaces) provides internet services for other parts of the operating system and for Microsoft Store apps.
If you wrote an operating system that depends on the internet, would you want that operating system to use your browser, or would you trust your competitor's browser to keep your best interests in mind? 😉
FINALLY in the OS choice in posting this - where is Windows 12 ?
Windows 12 has not been released. In fact, Microsoft has said nothing about Windows 12. You may read rumors and speculation about Windows 12, and that's all they are.