The device plugs into your OBD2 port and can connect via either WiFi Bluetooth, depending on the device that you purchase. I recommend the WiFi one. Why? Because it is faster than what a Bluetooth one is, there is less lag. There is a video below demonstrating the device in action. The WiFi device works with both Android and iOS, however, the Bluetooth one only works with Android, unable to work on any iOS device.
The device that most people get is the elm 327 OBD2 device off eBay. Prices vary, but here is a link to one – https://www.ebay.com.au/itm/ELM327-WIFI-OBD2-OBDII-Wireless-Car-Diagnostic-Reader-Scanner-Adapter-for-iPhone-/150875334132
I personally used Dash Command on iOS for my iPad and iPhone. In my VE, I permanently mounted an Android Tablet within the car which was pretty cool. Having more technical data about the car available at the touch of a screen is very handy, especially when you want to diagnose issues and look for faults (as well as clear them) within your vehicle. It helps diagnose issues like: