The tutorial on the official website is more straightforward, with more detailed documentation such as cesium native
I’m not sure what you’re asking. What do you want documentation on Cesium Native for?
All of our public API, i.e., public classes and functions, has documentation in the form of comments. We know this isn’t immediately obvious, and we have a Github issue open with the goal of making this more accessible: More obvious documentation for users · Issue #273 · CesiumGS/cesium-unity · GitHub
Is there any relevant API documentation, such as which interfaces to call when I want to perform coordinate conversion
If you want to perform coordinate conversion, you should take a look at CesiumGeoreference.cs and CesiumWgs84Ellipsoid.cs. There are comments detailing what each function does.