I don't understand, does this just inject the plastic into the mold? Where do I get the mold from?

You 3d print molds with a separate 3d printer.

It feels implied from the video and the pictures and the opening paragraph

> If you own a 3D printer, you've got hard plastics covered. . But what if you want to make something soft or squishy, like grip pads or a gadget enclosure?

> You can use existing resin molds or 3D print your own.