Electronic – Why does the human skin steal field lines when it’s very non-conductive in dry state

How do those emulate human touch?

Capacitive touch screens don't work from the supposed "aura" that some folk believe emanates from humans - they'll work with dogs, cats, medium sized rodents, pieces of metal and probably wet wood. Maybe they'll also work with high dielectric plastics too.

Capacitance is not given by gods to humans but is firmly routed in the physics of objects and the "styli" or pen has to be held in the hand and therefore a significant capacitive connection to the body is made (whether wireless or otherwise).

I suspect that wireless styli that use batteries may work by sensing the capacitive sensor's AC field and re-injecting some form of antiphase signal - the disruption is likely to be enough to "register" a keypress.

An easier way to understand this is to realize that when a copper plate is placed above the circular conductors it's essentially creating a capacitor with capacitance between the three conductors. The middle conductor will be floating at DC, but touch sensors are AC measurements so DC doesn't really matter. The diagram below shows three capacitors, but two of them are actually the same capacitance because of the outer ring.

The nice this is this capacitance can be estimated easily (there is some cross capacitance that is not shown, but the effects will be minimal from this), all you need to know is the distance between the plate, the area of the copper and the relative permittivity of the materials (or air) in between.

\$ C = \epsilon_0\epsilon_r\frac{A}{d}\$

Since there is already a small amount of capacitance between the plates (Cm) adding a copper plate above will create more capacitance and increase the current between the Tx and Rx plates. So a better way to look at it is instead of reflecting RF, you are increasing capacitance.

I haven't looked at the datasheet to know how the sensor data relates actual capacitance, I would guess that a higher sensor number indicates lower capacitance.

(Ctx and Crx could be modeled as a two capacitors in series which is essentially one capacitor so the values could be added together)