CesiumJS has a higher-level Entity API as well as a lower-level Primitive API. The API doc can be found here. I’ll point out a couple of interactive source examples that use the entity API; most Cesium apps use it. Cesium also provides open specification 3D Tiles to load, render, and cache on-demand terrain features, artifacts, and building renditions for surface viewing.
There is no IDE or download needed to interact with the material mentioned in this overview. You can interact with the material mentioned with your browser.