Taskwarrior has historically been a C++ project, but as of taskwarrior-3.0.0, the storage backend is now provided by a Rust library called TaskChampion.
TaskChampion implements storage and access to "replicas" containing a user's tasks. It defines an abstract model for this data, and also provides a simple Rust API for manipulating replicas. It also defines a method of synchronizing replicas and provides an implementation of that method in the form of a sync server.
Other applications, besides Taskwarrior, can use TaskChampion to manage tasks. Taskwarrior is just one application using the TaskChampion interface.
Taskwarrior's interface to TaskChampion is in src/taskchampion-cpp.
This links to taskchampion as a Rust dependency, and uses cxx to build a C++ API for it.
That API is defined, and documented, in src/taskchampion-cpp/src/lib.rs, and available in the tc namespace in C++ code.