$AMZNB is not available only through the Spot order book. Eligible users can also convert a supported Amazon stock position into the corresponding bStock.
Binance describes the conversion as 1:1 with no conversion fee, and the reverse route is available too. Service availability and regional eligibility still apply.
This is the bridge in a very literal sense: similar economic exposure can move between a stock position and an on-chain token instead of being locked into one format.