Im not a 100% sure what you mean by “can I make my own layers and add them on the paint so i can paint the mountains” but I am going to assume you mean vertex painting the same way you would on a static mesh, its not possible to do this on a Landscape. To paint textures onto a landscape you have to create landscape material and use the paint tool that’s in landscape mode. And you can make as many layers as you want but keep in mind that there is a limited number of textures that a single component can have (6 textures), if you go past this limit the component will turn black or have a checker box material.
Hope that helps.