AFAIK there are at least three types of touch sensor.
Resistive: Two electrodes very close to each other, so a finger can "short" them.
Many TVs in the seventies had them.
Mains hum: Mains hum from e.g a finger is amplified and detected.
An example is a touch lamp.
A 1Meg resistor from an analogue pin to ground, and a 1-10Meg resistor to a piece of touch metal is all you need. Code can detect mains hum and act upon it.
Capacitive: finger/body changes the frequency of an oscillator or the decay of a pulse.
These sensors can work through an isolator. 5mm thick glass is not a problem.
A large piece of grounded (to the circuit) metal around/close to the sensor could be needed.
Leo..