With pgzx we provide you a number of utilities on top of the C API. But depending on the extension you are working on you might heavily use the Postgres internals and their C APIs as is.
Postgres has support for a number of languages. Some having their own strenghts or weaknesses. One strength of Zig is its closeness to C, while providing a many quality of live improvements over C (optional types instead of null pointer, generic data structures, ...). This makes it a formidable choice for extensions that want to integrate deeply with the Postgres internals.