What makes an active buzzer oscillate?
There are 2 kinds of buzzers.

Piezoelectric ; which use internal CMOS Osc like 4xxx These operate like any Crystal Osc. Except the ceramic diaphragm is a large crystal.  All std ceramic caps in fact are piezoelectric but incapsulated.  These buzzers operate over a wide V+ range and use low current.

Schmitt Trigger Inverter oscillators are very simple using Dc negative feedback to self bias and with 180 deg phase shift , Ac Positive feedback to oscillate reliably with a square wave.  An extra inverter doubles the voltage across the resonator.
Since they use 4xxx series CMOS the driver impedance fairly high from 3k to 300 ohms at max voltage, they may be ganged in parallel for higher power.

Magnetic ; which use a relay type mechanism on the sound diaphragm that the internal coil current breaks from the metallic diaphragm , when actuated , so it relaxes by a spring and repeats . These take more current &gt;20 mA typ depending on dBa levels.