It's called a j-hook bracket, but I couldn't find that exact one.

This isn't quite what you're looking for, but it feels like it's on the right track, at least

Interlocking wall mounting brackets feels like a good place to start 🤷‍♂️

Thinking about this, and how I would ask someone how it works (it's on the underside of the shelf and has a J-shape with some kind of set-screw) makes me think this is meant to mount on two horizontal rails, one at the back of the shelf and one at the front. The rails would likely stretch across a large gap in the cabinet and be rod or bar shaped. To mount the shelf you probably hook it on one rail, drop it down and then use the set screw to lock it in place so it can't slide back and forth.