The mechanism for steering and also trying to keep the weight down on a bakfiets bike is tricky. But I don't think the mechanism is too difficult. Americans call Helm joints "rod end bearings"
I watched Phil Vandelay's cargo bike video, but he has a lot of neat projects. He sells plans. There is also a company from Germany that sells plans to make a bakfiets style bike using bolted aluminum tubing, which might be interesting for people that don't weld.
Don't remember the name right off.
Certainly a few hits for cargo trike on youtube
https://www.youtube.com/results?sear...cle+cargo+bike