I've worked through a couple different protocols for quantum key distribution, but when I look at the Qiskit textbook on this page I am unable to determine which one was used in this lesson. If anyone knows which one was implemented I would appreciate your assistance!
Looking into it, there are two main categories for implementations of Quantum Key Distribution. One of them is the protocol where you prepare and measure random quantum bits, and the other protocol which uses entanglement to connect two qubits where measuring one qubit will affect the measurement of the other.
The specific protocol that I believe is being described in the Qiskit textbook is the preparing and meauring protocol BB48, created by Charles H. Bennett and Gilles Brassard which you can find more information about here.
I also used this resource, so feel free to keep looking into it. I hope that helps!