I don't know what to call this technique, but how the make the overlay like on the bottom tier of this cake.?

I think you can roll it out and cut the scallops then lay the piece on top, or drape the whole piece then cut the scallop then apply the stripes. IMO, hope that helps

Wonder if a template exists?

I would just roll the fondant out in the size you want and cut the scallop design, it doesn't have to be perfect, I think it is more interesting if it isn't. Then I would add the strips of fondant, and cover the whole thing up with the border of fondant. I would invest in a clay gun for the ropes of fondant for the border, that way its consistent.