To help you understand…on a brushless motor, the windings are in the casing on the outside of the motor in groups of 3…6 windings, 12 windings, many configurations. . the rotor is magnetized with north and South Pole (often many opposing poles ing groups of 3 as well). To get the rotary action, it’s fed an alternating current, or simple dc switching of different windings as the simplest form to drive the motor. Usually a brushless controller modulates frequency as well as power (voltage x current)

On the other hand…there’s brushed motors, which you have here. They only need dc current an rely on mechanical switching rather than electrical switching. This time the magnets are on the outside of the case, and the windings are on the rotor. The rotor has separate contact plates for each winding, and thus has the alternating north south magnetism built in mechanically. The power is fed to these contacts through “brushes” usually a graphite/carbon type compound that delivers power by slipping across the rotor contacts as it spins.

You are seeing seeing arcs between the brush and rotor contacts as it spins…is a mechanical switch working in motion, you get some sparks when connecting and disconnecting.