At first glance this phase angle power controller has a strong case resemblance to the common 12V chassis power supplies. But this is a beefy triac based mains power controller for things like lamps, heaters or some motors.
It has a few notable features:-
Nice case, beefy triac (one of the biggest of its type if it’s original), extra circuitry to give greater stability and output symmetry at lower levels and a bit of effort to ensure that tracks are robust enough to handle fault conditions.
It does have two slight issues. A glass fuse instead of a ceramic one and a seemingly excessive resistor value in the snubber network. 4700 ohms as opposed tot he more common 100 or even 47 ohms.

Whether you should actually use a generic Chinese touch switch is debatable. They don’t seem to fully understand electrical isolation much. A version sold by a prominent local supplier is more likely to be built to a safer standard. This module came with four wire nuts, which are rarely used in the UK.

The bulk of the work on the PCB is being done by a dedicated chip that is not the typical microcontroller. It has dedicated circuitry including an amplifier and filter for the touch input and timing for the triac driver. The use of a capacitor in series with the triac gate is to keep the power supply simpler, as it results in a short trigger pulse of reasonable current.

The data sheet for the chip has extra layers of safety on the touch input, both for the user and the chip. The extra components have been omitted in this design. The timing resistor is an odd value. Possibly a “safe” value for both 50 and 60Hz that ensures the triac trigger points are well within each half sinewave.

The triac is very small. Its rating is a surprisingly high 600mA which suggests a typical lamp rating of 60W or much lower with LED lamps due to the high inrush current associated with their circuitry. The most common failure with dimmer modules like this is the triac going short circuit when a lamp fails with a pulse of high current.

With the current heatwave it seems a good time to take a look inside a fan controller.

As is very predictable with grey imports, it is very basic and lacks any electrical noise suppression. The triac also lacks a basic snubber network, which is usually desirable with inductive loads. Such a simple circuit may have stability issues with some inductive loads.

The spare component positions could have been intended for a couple of things. A crude locator with neon and resistor for finding the controller in the dark, or a snubber network.
If they were intended for a snubber network then it’s possible that it was left off because it takes very little current to make a fan rotate, and it may have leaked enough to keep a new fan turning slowly. If it had been on the other side of the switch it would have been less of an issue.