The advantage of Zigbee devices is that they are very low power, and they communicate in a wireless mesh network. The sensors are small and can work off a CR2032 coin cell for at least 2 years, maybe more. Depending on the type of sensor, they cost around US$10 and are readily available from various manufacturers, such as Xiaomi, Aqara (pictured below) or IKEA under their TRÅDFRI range of products.
You typically pair these sensors with a Zigbee gateway, which speaks the IEEE 802.15.4 protocol and relays the information (e.g. sensor readings) to either your mobile app or stores it in the cloud (or the gateway itself). But as you can imagine, a Raspberry Pi with the right adapter can do this job and offer more flexibility.