Lying on the bottom of every cabinet is a board that holds the transformer (upper left), fuses (lower left), score motor (middle left), and relays (along the right side). In the upper right corner are the connectors to the playfield and backbox. In the lower left is a small connector to the coin door. This is from a very simple game. In some games these boards are much more populated with relays and stepper units as you see in this next picture:

This is a top and bottom view of the same playfield. At the top of the underside there is a bank of relays that reset each time a new game begins, raising the drop targets and resetting target score values. There is also one stepper unit on the underside. All other components are bulbs and switches and coils for flippers, slingshots and pop bumpers.

This is what you might see if you opened the back door and looked inside the backbox. This is from a simple one-player game. A two or four player game would be much more densely populated with additional scoring reels and relays to drive them.

Notice what can happen when a powerful solenoid gets stuck on!!! (Clue: Look for black "hot spot") This solenoid is for one of the pop bumpers on the playfield. A stuck switch or relay would allow current to continuously pass through the coil generating plenty of heat. Occasionally a fire may result!