You are already using materials on the landscape which can do this, but it doesn’t alter the original mesh data (collision, amount of vertices etc.). What you are looking for is probably not a shader effect but a more advanced sculpting tool that lets you edit the mesh, or automates that for you (say, automated water erosion on rocks if A or if B).
If you go the procedural route you might skip the landscape editor for most part. Diamond square algorithm for large terrains with lakes and mountains.
2 algorithms you need, (also solved in the posts). There are more algos of course that generate things the landscape system might not even support, I believe that is just a flat plane? I use marching algorithms for most things (caves, indoor areas, some organic shapes).
Anyone familiar with the Diamond - Square algorithm? - #3 by UnrealEverything
Math in Delaunay triangulation algorithm, too many triangles appearing - #4 by Roy_Wierer.Seda145