While downloading the Windows Support Software (formally the Boot Camp Support Software) should be possible for any Intel based Mac from a M1 Mac, you would first have to at least know which Intel Mac and possibly which version of Windows.
According to everyman.com an Apple iMac 27-Inch "Core i5" 3.3 (5K, Mid-2015) has the Model Identifier
iMac15,1. According to Apple this model had the Magic Mouse option. AFAIK, the only Windows Support Software download available from Apple for an
iMac15,1 is from this link. This installs to the
/Library/Application\ Support/BootCamp folder. The download contains over 2700 items. I found a
AppleWirelessMouse64.exe that might be helpful.