How does resistance welding work?

Resistance welding joins parts by applying pressure and electric current with electrodes and locally fusing the pressurized area with the generated Joule heat.
Electric current is passed to the object to be welded through electrodes. Copper alloys are typically used for electrodes, so the resistance of electrodes is lower than that of the object to be welded. The temperature in the joined area rises relatively high, and only the object to be welded is fused as a result.
As the generated heat is concentrated in the joined area, welding with less thermal distortion is generally possible in a short time frame, compared to arc welding, etc.