flatbuffers are build according to a specific “schema”, so if you know the schema type of the flatbuffer, file, the answer here is to call FlatBuffers(FlatBufferType, bytes).
On a related note, there is the Arrow.jl repo that supports reading the in-memory arrow format (though I dont’ believe it quite handles all the IPC formats yet). In any case, that would probably be a good package to reference since it’s doing a similar thing to what you’re after for the in-memory structure.