This is a switch mode power supply. In order to use a smaller transformer it uses very high frequency AC rather than 50/60Hz mains frequency. In order to do th

This is a switch mode power supply. In order to use a smaller transformer it uses very high frequency AC rather than 50/60Hz mains frequency. In order to do this it rectifies the mains input to high voltage DC (330/220Vish, depending on the mains voltage) then using a power transistor rapidly switches this on and off at around 1-10kHz. Then the transformer outputs low voltage AC which is re-rectified and filtered to produce low voltage DC. The switch mode chip monitors the output voltage and modulates the switching frequency to maintain a constant output voltage. If you look closely at the board (and he talks about this in the video) there's a separation between the high and low voltage sides. Only the transformer and certain specialized components should cross this (Y-class capacitor and optoisolators). Cheap power supplies often miss this out which could allow a small bit of loose solder being able to send mains voltage to the output. Look at other videos by Big Clive to see the horrors. The high voltage side is a bit of a misnomer. It's really just the non-isolated side - the chip runs on 5v or similar, but it's not completely separated from the high voltage DC so isn't safe for human contact.