How do I download a DVD using Windows?

Download? From where? To where?

Downloading implies you are getting it from the internet somewhere, and want to save it onto your computer. But then it would not really be a DVD, just likely a video file.

If you mean rip a physical DVD inserted into a optical drive on your computer and save the files onto your computer, then you need an App that can do so.

If its a commercial movie DVD, then that would require breaking the DRM protection on it, and cannot be discussed here as that would be aiding in Piracy.

If its a personal DVD with home videos, and no DRM protection, you can use Apps like Handbrake.

https://handbrake.fr/