A crystal is one arrangement repeated over and over. The smallest piece of that repetition is the unit cell. Pick a structure and one unit cell is drawn in 3D.
Press one of the figures in the properties panel and the counting is played out on the model itself. Press an atom and you see, as a fraction, how much of it belongs to this cell.
Drag to turn the model and use the wheel to zoom (on a phone, drag with one finger and pinch with two). Atoms can be drawn small, which makes the positions easy to read, or touching, which makes the relation between radius and edge visible.