Start your Adafruit nRF52840 board with code running from code.py. You must have code running for this to work! The examples on the following pages have ready to go CircuitPython code.py scripts to try out.
Next, start up the Bluefruit LE Connect app, and make sure it's in Central Mode (left button on the bottom). When you start the app, you should see a device named CIRCU or CIRCUITPY. If there's a long list of devices, you can shorten it by turning on the "Must have UART Service" switch.
To connect to the Adafruit nRF52840, touch the Connect button. You should see "Connecting" and then "Discovering Services".
If you don't see CIRCU or CIRCUITPY right away, try pulling down to refresh. If that doesn't work, try turning Bluetooth off and back on on your phone or tablet and restarting the app.
After you connect, you'll see a menu of how you can interact with the device. Choose Controller.
Here's the Control Pad screen. You can use the buttons as inputs in CircuitPython. For example, you could have code that moved a servo arm up and down when the up or down button is pressed.
To change the color of the NeoPixels, use the Color Picker screen. Choose a color and press Send Selected Color.
There are multiple options under Stream Sensor Data including Accelerometer. You enable these options to stream the associated data from your mobile device to your Adafruit nRF52840 board, and use CircuitPython to read that data.
For example, if you have code that reads the acceleration data from your mobile device, you'd want to enable Accelerometer once you connected to the app.
Check out this detailed guide for more information about the Adafruit Bluefruit LE Connect application.