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I didn't examine the sensors but I'm pretty sure one part was magnetic and the other part was a reed switch. The magnet holds the switch closed (conducting) while the window is shut. The circuit is a loop of closed switches connected around the entire house. Probably a few volts and a few mA.

Someone opening any door or window moves a magnet away, causing a reed switch to open. This breaks the circuit. The central board detects this and activates the alarm.