This is a basic support for the underside of a shelf using a 4x2 Multigrid baseplate: https://thangs.com/designer/Keep%20Making/3d-model/4x2%20Multigrid%20Base%20Plate-973868

The longer sides (the side with the screw hole) should mount into the board. For heavy loads, if you have concerns about the shelf become dislodged you can mount it with these: https://thangs.com/designer/Keep%20Making/3d-model/13%20mm%20Big%20Thread%2C%20Flat%20Push-Fit%20Head%2C%20Small%20Thread%20Hole%2C%20Bolt-973876 by screwing small screw into the back of the piece before attaching it to the wall. (see make by user mase711). You can screw the whole assembly in until you can push the bottom in the connector below. (thanks for the tip, mase711). Alternatively you may choose to use a tight snap as well https://thangs.com/designer/Keep%20Making/3d-model/Multiboard%20Tight%20Snap-974403