Where can I find the data structure of 3dtiles?
The spec is here: https://github.com/AnalyticalGraphicsInc/3d-tiles
As for texture compression, I saw the article about Draco compressed meshes with GLTF and 3Dtiles on the website. Can Draco compress Bentley’s cesium data ?
Draco should be able to compress any mesh or point cloud. You would need to apply it to each of the models in the tileset. For example, gltf-pipeline has support for compressing a gltf model.