how i can add new mats in the layer

Hello, im doing a map with diferents mats. I have downloaded different mats like snow or sand, but i dont know how to put them directly in the site of Target Layer -> Layer.
I want clic in the mats to draw some region but not all map and i dont know how to do that.

So you were wanting more materials on one terrain and switch to which one you want to apply am I correct?

what type of objects would be the first question. Generally, a static mesh has Material ID’s defined in an application for 3d modelling and texturing. Mes PAing using Vertex Colors is probably the best way to go about this: