To mitigate the issue of cross-technology interference (CTI) under dense wireless, cross-technology communication (CTC) was recently proposed, which enables direct communication among heterogeneous wireless technologies. We present SymBee, a novel ZigBee to WiFi CTC with symbol-level encoding for performance breakthrough from packet-level state-of-the-arts. SymBee is uniquely built on the new insight on ZigBee-WiFi physical layer cross-observability - i.e., the output on WiFi when fed with ZigBee signal (due to frequency overlap). This is analyzed experimentally and theoretically through rigorous derivations, from which the key innovation in SymBee design, i.e., payload encoding, stems; Conveying data across technologies is as simple as putting specific symbols in ZigBee packet payload, such that they yield unique and easily detectable patterns when cross-observed at WiFi. This symbol-level encoding is fully compatible with any commodity ZigBee device. Decoding at WiFi is a light-weight function that recycles the output from idle listening, thereby minimizing the computation while keeping compatibility to WiFi standard. SymBee is extensively evaluated both theoretically and experimentally through testbed evaluations on six distinct locations including outdoor. The result demonstrate that SymBee reaches the throughput of up to 31.25kbps, 145.4× faster than the state-of-the-art.