As this 2008 YouTube video by Matt Rittman shows, there's some surprisingly sophisticated stuff going on behind the back wall of the alley. When your ball hits the pins, it (along with, ideally, the knocked-over pins) falls into a collection area behind the deck. The bottom of the collection area is a conveyor belt that sweeps the pins back under a low barrier; because the ball can't fit under the barrier, it finds its way into a separate hole.
The sweep — that bar that grabs knocked-over pins — comes down; the pinsetter grabs the still-upright pins, and the remaining pins are swept back to the conveyor belt. Meanwhile, the ball gets pushed up a very steep track via a "ball accelerator"; once it reaches the top of the ramp, it returns purely by force of gravity.
The pins are brought to the top of the device via an "elevator wheel" — essentially a wheel with little scoops designed to grab them (via Matt Bowlin, also on YouTube) — and dumped into a "pin distributor," which puts them into the pinsetter's holes via a system of tracks and funnels. Pins are bottom-heavy, so they always come out upright (via How It Works). Then, after you throw your second gutter ball of the frame, the pins are lowered back where they go.
So there you have it. Think about all that next time you're wearing ugly shoes and choking on decades-old cigarette smoke. Or the next time you go bowling.
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