[moqmar]

The DC side is fully isolated except through a capacitor that is there to reduce EMI interference and is specifically built to "fail safe", except for the cheapest no-name imported power supplies. (https://www.pcbaaa.com/y1-capacitors-function-application-an...)

[MadnessASAP]

A Y capacitor prevents the neutral from shorting to ground, in this case Apple has cleverly avoided the issue by not having a ground whatsoever.

I am however curious about your design for a galvanically isolated AC-DC power supply.

[grues-dinner]

That's not what the Y capacitor is doing here. All Y means is "high voltage withstand, will fail open, and we have receipts". Here, it's not line-to-ground, but primary-to-secondary.

Isolated ACDC converters are very common these days. Companies like Traco will sell you modules: https://www.tracopower.com/isolated-power-supplies