Does Magic Mouse automatically sync up with Windows in Boot Camp? I may be just going slightly mad, but hear me out.
I have a Macbook Pro running Catalina. My Magic Mouse 2 has been paired to it for a while.
I just installed Windows 10 to dual boot it using Boot Camp.
Now, I just got out the mouse thinking I needed to pair it again in Windows, but then I noticed it was already paired and working automatically! I really do not recall pairing it in Windows. I could be awfully forgetful and already did it, but I really do not remember doing so!
Did Apple make some feature in the Boot Camp tools that are installed in Windows causing bluetooth devices such as MM being synced between macOS and Windows?
Once the wireless mouse is paired to the Mac, the mouse will work with Windows, the Mac Startup Manager, Mac Recovery, etc. Specific to your question, the Boot Camp Assistant restarts the Mac to boot the Microsoft Windows installer in such a way as to keep the mouse paired with the Mac. When installing Windows without the aid of the Boot Camp Assistant, often (but not always) a user has to plug in a wired mouse to install Windows and pair the wireless mouse after installation completes.

Unless you have installed Windows without the aid of the Boot Camp Assistant, you never realize how many steps are automatically done for you.