I would recommend looking at the Civil 3D .NET API Reference in conjunction with looking at Python scripts developed by other users. Try to take note of specific classes and methods they are using, understand the relationship between them, and try to understand the general workflow.
Once you understand how to get the native objects from C3D into Python (see here for an example), it’s just a matter of having a better understanding of the C3D API to really understand how you can manipulate the data.
@Paolo_Emilio_Serra1 has provided various Python snippets on the forum as well if you look through his post history.