You need some kind of sensor to sense the touch. Various kinds have been used; a microswitch, electrical, piezo-electric, load-cell. Marlin supports any of these... they just provide a digital on/off input to the controller.
I have a cheap AliExpress printer which came with the microswitch. The nozzle has to move (.5 mm or so?) to activate the switch. It's sprung to return it to the same place when the pressure is released when the nozzle is lifted. So there's a small Z distance between the "normal" and "touching" positions of the nozzle. On my printer, the spring wasn't strong enough, and the hinge mechanism also allowed some lateral movement. All of that led to a lack of repeatability in positioning the nozzle. I really think that this is more trouble than its worth, and I fairly quickly threw out the whole auto-levelling thing and just manually level the bed and set the Z=0 height.
Electrical relies on measuring the resistance between nozzle and a metal bed... when it's zero, the nozzle is touching. Obviously you can't use this if you print onto glass or tape. I don't know how well this works, but you can imagine what happens if there's some filament stuck between nozzle and bed. I don't think many people use it.
The other 2 are (relatively) expensive and newly developed or even experimental; I'd be surprised if you got them on an AliExpress printer.
I have a cheap AliExpress printer which came with the microswitch. The nozzle has to move (.5 mm or so?) to activate the switch. It's sprung to return it to the same place when the pressure is released when the nozzle is lifted. So there's a small Z distance between the "normal" and "touching" positions of the nozzle. On my printer, the spring wasn't strong enough, and the hinge mechanism also allowed some lateral movement. All of that led to a lack of repeatability in positioning the nozzle. I really think that this is more trouble than its worth, and I fairly quickly threw out the whole auto-levelling thing and just manually level the bed and set the Z=0 height.
Electrical relies on measuring the resistance between nozzle and a metal bed... when it's zero, the nozzle is touching. Obviously you can't use this if you print onto glass or tape. I don't know how well this works, but you can imagine what happens if there's some filament stuck between nozzle and bed. I don't think many people use it.
The other 2 are (relatively) expensive and newly developed or even experimental; I'd be surprised if you got them on an AliExpress printer.