An EFP works on the same principles as the warheads in anti-armor rockets and missiles.
The explosive material is in shaped charge form behind the copper plate.
The explosive produces a plasma jet when it detonates and that plasma jet superheats and cuts through metal armor plate. The copper is formed into a molten metal slug by the same explosion and follows the plasma jet.
The slug smashes through any heat resistant ceramic in the armor and also disintegrates into a multitude of various sized metal pellets once it penetrates through the armor.
The killing effects are:
Exposure to the primary plasma jet.
Explosive overpressure inside the vehicle.
Heat flash intense enough to ignite ammunition and/or fuel, as well as burn flesh.
Shrapnel from the disintegrated slug and/or armor spauling (fragments of the armor plate turned into shrapnel by the impact of the warhead)
With the price of copper so high, many low-paid insurgents would do better to sell it for scrap.
They’d only have to desert, accept their whole family as dead, and then flee the rest of their lives.