[eli5] How can you dial 911 anywhere in the US and still get the police closest to you?
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This works a few different ways, but relies on Automatic Number Identification and Automatic Location Identification (ANI/ALI).
Landlines: If you call 911 from a landline, the state 911 system already knows exactly where you are because your landline number is associated with an address. The call is then routed to the appropriate Public Safety Answering Point (PSAP).
Cell phone, Phase 1: When you call 911, your cell phone connects to a cell tower. That cell tower will route your call to the closest PSAP to its physical location. This method works most of the time. But sometimes if you are near a jurisdictional boundary, you might connect to a tower in a different county and be sent to that PSAP. However, they can usually transfer you to the right place.
Cell phone, Phase 2: If you have E911 location service activated on your phone (which comes standard on newer phones), it will use that to determine where to send you. When you call 911, your phone will use GPS to figure out where you are and send your coordinates to the cell tower as well. Your call will be routed to the PSAP based on the GPS coordinates on your phone. If service is too slow to transfer GPS data, or GPS data is unavailable, or that feature is disabled, it will revert to Phase 1.
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