Initial direction of a motor
How are those small DC motors always rotate in one direction unless we
reverse supply's polarity?

For a DC motor with two commutator contacts (one rotor coil) the motor can run in any direction so let's clear up that first. What makes a practical DC motor run in the same direction all the time (for a given supply polarity) is that there are at least four commutator contacts and therefore two rotor coils. Let's deal with that situation.
The commutator brushes are positioned so that they only energize the rotor coil that is physically more biased to align its magnetic field with the stator magnets by rotating in (say) an anticlockwise way. So it begins moving anticlockwise to make an alignment.
90° of rotation later, the brushes are no longer driving the first coil because the commutator has rotated with the rotor; they are driving the 2nd coil and it's the same scenario. That new coil is physically more biased to align its (new) magnetic field with the stator magnets by rotating in the same anticlockwise way.
The process starts over.
More commutator contacts means more rotor coils and the angle at which the nth coil stops taking current and the n+1 th coil begins taking current becomes smaller. So, 8 commutator contacts means the rotor has 4 coils and changeover occurs every 45°.