The power is still running through the alarm panel, which in most cases outputs 12V DC. If you have an existing alarm panel, you can easily draw power from it to power the NodeMCU. The wires on the left side of the alarm panel contacts are mostly for power out. My siren, motion sensors and glass-break sensors are still powered through the alarm panel, but the sensor/trigger wire has been moved over to the NodeMCU.
The wires on the right side of the alarm panel are for the analog phone line. I just never disconnected them, just lazy. It doesn’t do anything … I don’t even have landline phone service.
If you want to completely remove your old alarm panel for the “cleanest” installation, you can totally do that. In that case, you should power the NodeMCU device with a 12V DC power adapter and you can use it to send power to the motion detectors, siren and other powered sensors throughout the house.
See here for more detail on power: