It's all manual memory management in the FFI layer, driven by the semantics of the type expressions. The type expression syntax is compiled to a tree of objects. Those objects are walked before and after a FFI call to do the right actions related to memory management, encoding and decoding. Any temporary buffers needed by FFI are cleaned up by it. The Lisp objects going in and out are stock Lisp objects without any special memory management related to FFI.
How does memory management work? Do you have self-destructing structures (AKA automatic pointers) on the Lisp side?