Moderately experienced bike wrench looking to build adjustable tall bike

For starters, let me link to a video of what I'm trying to accomplish.

That bike is not sold by any company and was built custom made. Based on the video and digging around the internet, it looks like this was accomplished by gas springs on the rear triangle, which is definitely something custom built.

I'm still trying to figure out how the rear triangle is able to adjust itself without being welded in a specific position. Unfortunately, there isn't a clear close up of the bottom bracket or the rear triangle.

If anyone here has any other observations, advice, suggestions, I'm all ears.