I saw a circuit in the book, but I don't quite understand what role the part in the box plays in the circuit?

It is marked in the figure as an active load (constant current) amplifier (common emission)

Is there a contact on the left side of the 330p? Where did you see this picture?

The differential amplifier detects the change in the output voltage to adjust the output voltage.Any form of voltage regulator circuit is the same, but the sampling and amplifying circuit is different, this basic thing.

According to the data of the differential amplifier principle, the simplest way is to generate the circuit effect through the common emitter resistance. For example, the output voltage increases, the last transistor is amplified, which causes the emitter resistance voltage to rise and the second transistor base voltage regulator tube voltage. By comparison, the collector voltage drop caused by the decrease of the current of the last second transistor is changed-the subsequent circuit is continuously adjusted.