There are many types of sensors available to HomeKit: motion, vibration, temperature, humidity, air quality, contact, leak, smoke, etc. Many sensors can detect more than one condition, for example humidity and temperature. You can use sensor information for a variety of things in HomeKit, including automations such as turning on a humidifier when the humidity drops below a certain point, or turning on a hallway light when somebody approaches a hallway. Sensors may come in standalone sensors, or as part of another device, such as a thermostat, doorbell, or security camera.
Bluetooth: Bluetooth has limitations such as speed and distance, but it is inexpensive to implement and can last a long time on battery power, which can make them smaller and easier to install, making them well suited for small sensors.
Zigbee: Zigbee devices can respond quickly and can also last a long time on battery power. Zigbee sensors will need a bridge to connect to HomeKit.
Wi-Fi: Wi-Fi offers has advantages over Bluetooth, including connectivity over longer distances, but will also require a power source for practical usage. This typically makes Wi-Fi sensors larger.
Some motion sensors only detect motion, but most of them contain other sensors as well. You will often find a motion sensor paired with a light sensor; this allows you to tale different actions when motion is detected based on the light level. Another common sensor pairing is motion, temperature, and humidity.
Most sensors that are measuring air quality values measure more than one thing. You will typically find at least temperature and humidity in a single sensor, and more often than not, they will also measure particles in the air and give you a "air quality" score.
The Ecobee SmartSensors provide temperature and occupancy detection
Onvis Smart Motion Sensor - The Onvis is a handy little Bluetooth 5.0 motion sensor that also measures temperature and humidity. It stores 31 days of data, and uses two AAA batteries that last close to two years. I have found them to be reliable, although they need to be relatively close and line-of-sight to a HomeKit Hub.
Koogeek Smart Environment Monitor - The Koogeek Smart Environment Monitor specifications state: "6 high precision sensors and 360° open air circulation design ensure the accuracy and sensitivity". It has a 2.8'' LCD Screen that displays all of the readings it can measure: relative humidity, particulates (PM2.5 and PM10), as well as motion detection, light intensity, sound levels, the current time, charging status, and battery levels. It connects over Wi-Fi and uses a rechargeable battery or can be plugged in using the included adapter. A couple of operation notes: This unit has a fan in it (the manufacturer indicates that this is to get more accurate readings) which makes a quiet white-noise type of sound. I would not recommend it on a night stand or near a reading nook due to the noise. I have also seen the motion detector trigger when I know that nobody was in the room. If I were to purchase another device like this (something that measured temperature with a display that could be plugged in) I would give the Eve Room Indoor Air Quality Monitor a try.
First Alert OneLink Safe & Sound Smoke Detector