The OpenZGY C++ API allows read/write access to files stored in the ZGY format. The main part of the API is here:

IZgyReader and its IZgyMeta base class.
IZgyWriter and its ZgyWriterArgs argument package.
IZgyUtils for anything not read or write.
List of exceptions that you might want to catch.
SeismicStoreIOContext for cloud credentials.
ProgressWithDots example of progress reporting.
Example Example application.

If you are viewing the full Doxygen documentation then this covers both the API and most of the implementation. So if you look at the list of classes and methods this might seem a bit daunting. All you really need to use the API should be in the above list. Excluding trivial structs that will be cross referenced as needed. So you don't need to go looking for them. Of course, if you want to work om OpenZGY itself then you probably need everything.