Motor Voltage and Current Limit of Brushed DC Motor
For most types of electric motors, the speed is proportional to voltage or voltage and frequency. The maximum voltage and frequency is limited by the winding insulation, brush/commutator arcing, mechanical speed limitations and magnetic properties.

The impedance is not constant and is not primarily the winding resistance. With no load applied, the effective impedance is high and the motor runs at the highest possible speed for the applied voltage and frequency. The current drawn is low. The electrical power is all being dissipated in resistance, mechanical friction and other losses. When load torque is applied, the speed starts to decrease and the effective impedance decreases allowing current to flow and electrical power to be converted to mechanical power delivered as driving torque and speed.

The maximum driving torque is limited by the area of the air gap and the strength of the magnetic field between the rotating and stationary parts of the motor. The magnetic circuit has a limit to the magnetic field that can flow. In addition, there is a thermal limit to the current that can flow.

The above is a very general and simplified summary. This forum is not suitable for describing all of the details for even one of the many types of electric motors. You need to study text books, course notes or other detailed explanations that can be found online.