Ah, so THAT'S what they base vtuber neck lengths on!

It doesn't elongate the neck it actually shoves the shoulders and collarbone down. That's why they look a little like a wide based palm tree. The clavicle and scapula stay In place by the first rib but along the spine.

I looked up an x ray because none of the relevant articles had one

I'm far from an expert, but it seems that not only does it straighten and elongate the neck, but it also pushes down all the bones around your shoulders.

The neck cannot stretch. This is displacing the bones around the scapula and shoulder down, and neck muscles will atrophy.