Why does the Aurora D8 have vents on the bottom of its fuselage?
If you're talking about the dark spots here...



...then those are the tires of the main landing gear. This is the same approach used by the Boeing 737 — as discussed in another question here.

By skipping main landing gear bay doors, the aircraft's total weight is reduced. An aerodynamicist could provide a more confident answer, but the impact of the exposed wheels to the airflow would be relatively minor and not impact drag meaningfully.