Jio Wireless Door / Window Magnetic Sensor For Home Security 

This sensors consists of itself and a little magnet that's attached to a door or window. It sends a state update when it's opened or closed. These sensors can be used to trigger automations when the apartment door is opened as well as for general state monitoring.

These sensors communicate over the Zigbee protocol. Zigbee uses a 2.4GHz band that's similar to Wifi. It's main advantage is however that it forms a mesh network. So a tiny sensor running on a button cell doesn't have to connect at a central hub but rather uses the nearest mains powered device to repeat its signal. Since most of the devices are also controlled over Zigbee I in any project have a fairly extensive Zigbee mesh and these sensors never need to broadcast very far. In general the button cell of most of the sensors seems to last between 1-2years before needing to be replaced.