How to create a bootable installer for macOS from windows

Not possible

To perform this action will require booting from a Bootable Installer. 

This will have to be performed from a Qualifying Apple Computer to run the version of macOS to be made on the Bootable Installer.

Example : Bootable Installer of Big Sur would have to be done on a computer that Qualifies to run Big Sur.

As was mentioned, it is probably not possible. Aside for copying the macOS files, the volume has to be "blessed" so it can boot. Best to find a friend that has a Mac to create one. It requires a 16GB USB key which can be easily and cheaply sent by mail (i.e. actual mail). It is also possible to create a bootable optical disc (but this is another topic).

I also wonder if an Apple Store might be willing to assist you in this regard.

- Pie Lover

You can't.