uA702 OpAmp internal circuit confusion
As the paper explains, Q3 functions as an opamp in over a limited domain of operation in the sense that its output (collector voltage) is a function of the difference between its two inputs (base and emitter). Negative feedback via R1 insures that the base (inverting input) is held at a relatively fixed voltage with respect to the emitter (noninverting input).

This assists the rest of the circuit by making sure that all of the signal voltage variation across R1, which is caused by the current variation in Q1, appears at the upper end of R1 rather than at its lower end, where it is connected to Q1's collector. This means that the signal voltage is also imposed on the upper end of R2, where it can help drive Q4 and the rest of the circuit.

Q3 has essentially the same function as the upper transistor in a cascode amplifier, holding Q1's collector at a fixed voltage, but via a different mechanism.