Tapping into a 5.1V transformer for a zero-crossing detection circuit
You don't tap into the AC anywhere.

That type of power supply doesn't have the AC signal from the line voltage at a lower level.

It converts the line voltage AC to high voltage DC, and then uses a high frequency signal to make pulsing DC which that little transformer can bring down to a lower voltage.

There is no low voltage point where you can get a zero crossing related to the line voltage zero crossing.