Hi @Haadi_M1 thank you for posting to the CesiumJS Community forums. I covered a bit in your other thread re: resources you can use to tile DSM data → CesiumJS CZML for real time data - CesiumJS - Cesium Community
You are correct that Cesium World Terrain is not a DSM as it is a DTM. There are free/public domain DSM resources out there like ASTER GDEM, ALOS World 3D or Copernicus DEM but those data sources are roughly 30m per pixel. Too coarse to represent buildings.
You could pull in our global OSM buildings from Cesium Ion. Or if the AOI you are working w/ is in Japan, Plateau 3D buildings.
Do you have a specific location in mind?