Hi Fantam,
So it sounds like you are somewhat new to the engine, so the way I understand your question is along the lines of understanding material instances.
Material instances can reference all parameters defined in the parent material they reference. Although I didn’t see material instances in the vertex painting video, I do see that the video uses texture sampler nodes for the grass and hex tiles. These are not parameters. When you right-click in the material editor, you can find the texture sampler parameter node. But if you already have your texture samplers nodes in the editor, you can simply right-click the node and there will be the option to convert it to a parameter. You will need to name it and this will show up in the material instance after applying the changes.