When you buy the Product Key from the Microsoft Store, the Download Manager (.exe) when run allows you to download an ISO image. An ISO image is a software disk image of a physical CD/DVD which can be burned to a physical CD/DVD depending on size.
From the Microsoft link...
When you download Windows from Microsoft Store, you have two options: You can download a collection of compressed files, or you can download an ISO file. An ISO file combines all the Windows installation files into a single uncompressed file.
If you choose to download an ISO file so you can create a bootable file from a DVD or USB drive, copy the Windows ISO file onto your drive and then run the Windows USB/DVD Download Tool. Then simply install Windows onto your computer directly from your USB or DVD drive.
When you download the ISO file, you must copy it onto a USB or DVD. When you're ready to install Windows, insert the USB drive or DVD with the ISO file on it and then run Setup.exe from the root folder on the drive.
On a Mac, once you have the ISO file, you can burn it to a physical DVD using Disk Utility (Yosemite): Burn a disk image to a CD or DVD.
