99.99% of HO uses Kadee couplers or clones.
The best way to describe how HO couplers work is with the Kadee Handout. These couplers uses a magnet (or an electro-magnet) either between the ties or under the ties. As mentioned, a skewer or small blade screwdriver can also be used for manual uncoupling.
I think MTH offered the only HO electro-coupler mounted on the locomotives.
Rusty