I think that similarly to how you can paint multiple textures in Unreal engine for terrain, one of the materials going into the terrain, is an animated material that been painted applied or projected to the Game map from from a stark top down view. that, or they are using animated Roughness and specular maps to create a wet map for the Game object’s materials. Such as for rocks and buildings, probably with a looped animation flip book , Or they could have a specular map with a looped animated normal, to give the illusion of water flowing. This Is one thing that came to my mind after climbing some sharp angled mountains and observing how the wet parts completely cut off at dramatic angles and create some unsightly seams in parts of the game where players are unlikely to visit. They could have some kind of expression set up, where the wet material is only displaying on object surfaces that have angles of less than 70 or 80 degrees. I haven’t tested these things out myself but these are my best educated guesses on how you might approach this.