UBECs are a "noisy" way of stepping down voltages. They can have quite high peek to peek voltages (which average out to 5V) but these peeks can damage things if not smoothed out.
The full size Pis have voltage regulators on the micro-USB power input which smooths out this peek (not on the GPIO though!) but the Pi Zero's don't.
The issue with a DC/DC voltage regulator is that the "extra voltage" is dispersed as heat. So they can get a little warm.
But to answer your question... this :)
This is an OKI-78SR-5/1.5-W36H-C DC/DC regulator! They're highly efficient ~90%, and provide 5V @ 1.5A (7.5W) from a 7V to 36V input.
Unlike noisy UBECs, this regulator provides < 75 mV peak-to-peak ripple.
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