The system here monitors temperature, humidity, and smoke levels and alerts you if those levels reach a certain point.

If you want to get started with motion sensors and the like, ESPHome makes it incredibly easy to build your own sensors.

The Primo Core is built around the nRF52832, and adds humidity, temperature, 3-axis magnetometer and a 3-axis accelerometer to a square inch of fiberglass.

Monitor temperature, humidity, and air pressure in every room with the Aqara Temperature Sensor. Optimize comfort, prevent damage, and automate your HomeKit smart home.

Enable the HomePod 2 and HomePod Mini's built-in temperature and humidity sensors and turn the devices into indoor weather stations.