In Unity 2019.1 how do you make the simplest mesh by script? You watch this video that's how. My Daughter and I go thru explaining how to make a triangle mesg (the simplest mesh) in Unity. We start from scratch and we add a texture. We explain uv mapping, backface culling, clockwise triangle points and crack a couple of jokes.
YouTube: https://youtu.be/J4kTo2Fb-XQ