Thinking more about your very good question, I have to conclude, Defender has to be scanning anything and everything on hard disk which is executable. So that would include any logical partitions on hard disk such as B:, D: etc.
My attempt here was simply to make Defender easier to run for all Windows-10 users. I do not claim to change or improve on how Defender operates.
And now, a more difficult question. I have three physical disk drives with two partitions on the first, creating C: and D:. The others are E: and F:. G is DVD and H: is my USB3 backup drive.
Any thoughts?