If you don't maximize the photo, it will be easier to see
There is no choice with this forum.
Would the "ground" be "the Earth" in this case?
In Arduino, capacitive sensing only works when you connect it to the Earth somehow
No you can get capacitance between any two connections.
The arduino capacitance sensor method is not very good, it is only a sort of demo.
To do it properly you need a controlled layout and an oscillator that changes with capacitance then you measure the frequency.
The pad seems to be made of metal
So that could be an inductive injection. Or it could be capacitive.
I have a similar dimmer with a plastic coating.
So that would be capacitive.
It is extremely hard, if not impossible, to tell what a circuit is doing by just looking at the outside case.