3D is achieved by rendering 2 frames from slightly horizontally view points simulating the position of each eye.
VR headsets than act as two screens which each of these frames are sent to. Note depending on the headset it can be 2 actual panels (LCD, OLED etc) or a single panel effectively split in half to act like 2 panels.
Since the original Oculus headset the addition of special lenses (Fresnel) allowed headset makes to use smaller panels which the lenses magnified. However these lenses added distortion, which had to adjusted for when rendering the picture. Fresnel lenses also give you more of a feeling of distance, old school vr headsets were just square panels that felt very up in your face.
So from you PC's point of view it is rendering a left eye and a right eye then having to apply fresnel correction. Quite a bit of overhead.
Also worth noting, if you want to get really technical. It is a simulation of 3D, not real 3D. It is rendering 2 flat images at a focal length of about 2m (I think). Which is why everything is in focus no matter the distance.