Standard non-insulated connector crimper. Nothing special.
I worked for Cummins years ago and their engine harness branches were done in the same manner. In fact, the factory 5.7 harness had a few in it as well.
I’ve seen plenty of people bash soldering of harnesses over the years. I chalk it up to them not knowing how to solder properly. Solder can most certainly wick up the conductor under the jacket, but you would have to be heating the **** out of the wire. I put heat on the buttsplice and the solder draws into the voids. Kinda like soldering copper pipe. The heat shrink supports the wire where it exits the buttsplice so there isn’t a hard spot for the wires to bend and break.
I’ve done this method for splicing and branching wires on cars since I played with car audio systems in high school 30 years ago. I’ve never had a solder joint fail. You know what I have seen fail, butt splices. Although those are usually from people not using the correct tool to crimp.
Being an electrical engineer and having worked in the marine market, I’ve seen my fair share of wiring failures in both AC and DC systems. I’m particularly anal when it comes to wiring.