Here’s my bench, built many years ago. I’ve made some drawings to illustrate how the design works. It’s pretty straightforward but feel free to ask questions.
The base stretchers are held in tension by truss rods in channels running through the legs where adjustable nuts allow dis-assembly and hold the base in a very rigid alignment. The legs are mortised into the bottom trestles.
I had a Emmert vise on here at one point as a tail vise. The end cap is thus mutilated.