Running through the circuit...
There's a brown wire (unfused permanent live) to the rocker switch. In position 1 it connects that supply to a solid red wire which supplies the side, tail & dash illumination via a fuse in the fuse box. Rocker switch position adds connection to a solid blue wire that goes to the headlamp stalk.
In the stalk's middle position it is connected to a blue/red wire that supplies both dip beam filaments. If only one is lighting then it's likely the bullet connection behind the grille, where the circuit splits to left & right, or the earth of the faulty light.*
Pushing the stalk forward connects the blue supply wire to blue/white for the high beams. It also supplies the blue warning light on the instruments which branches off the circuit where the stalk plugs in to the loom. So the H/B light can shine blue if the actual high beams don't even manage candle yellow.
Pulling the stalk ( headlight flash) connects the blue/white high beam to an purple wire (fused permanent live) so that it works regardless of the rocker switch position. This is also the supply for the horn push.
*as the earth is the same for the for dip & high beam, it can help diagnosing faults...
No lights at all on one side: probably the earth. Dip works, full beam doesn't: probably the fault is on the supply side.
Similarly, if the headlight flasher works but both headlights don't it's probably the rocker switch (and vice versa).
Finally, poor earths can give hours of fun as electrickery has a habit of finding its own route. E.g. if your left headlight had no earth on dip beam the current would try to flow through the shared earth connection and back through both high beam filaments to the earth on the right side, or via the indicators if their earth is common. As mentioned, this shouldn't be possible with LEDs (D for diode), but replacement led bulbs usually have extra gubbins to adjust the voltage and/or resistance to make them compatible.
