Classifying data can be as complex as you like, depending on your requirements. These days there is a lot of enthusiasm for machine learning, but that requires a training data set.
The place to start would be to set up the Nano BLE Sense to collect accelerometer data, and simply take a look at those data for a variety of human activities. The Serial plotter option (on the Arduino Serial Monitor) can display the three accelerometer variables as a function of time.
Edit: it is useful to display and consider the magnitude of the acceleration vector as well as the components, as the axial directionality is eliminated.
If ax, ay and az are float variables,
float amag = sqrt (ax*ax + ay*ay + az*az);