There's probably a black spot on the underside to connect the traces on the PCB.
It's been called many things, try looking for conductive carbon buttons or some combination of conductive/dome/carbon/silicone/squishything.
It's pretty low tech stuff, just open traces that detect when they're connected. A moist finger would work or a piece of metal, conductive thread, whatever. The dome gives some tactility, but more importantly has a layer of carbon on the underside to do the conducting.
Some people call them capacitive buttons, but that's typically used to refer to the ones without a mechanical button.