I think that centre is mass weighted, for both the BB and the camera's focus. You can make a long, light weight protusion on a little heavy tech and it doesn't move out to the middle by total width. Not certain it's precisely the COM, though.
Camera focus = Center of mass
Build beam center = Center of footprint
How to prove: build a long line out of Venture blocks
On one side, build up/down with Hawkeye blocks
Build beam center should stay the same, but camera will change focus as you place blocks