A magnet blank arrives unmagnetised or uniformly magnetised. A pulse magnetiser discharges a large current through a small coil held against the face. That pulse produces a brief, very intense field in a confined area, which sets the polarity of that region. Move, change the direction of the pulse, repeat. The pattern builds up region by region.
What limits the smallest maxel
Chiefly the coil and the energy behind it. A smaller coil writes a smaller region, but the field has to stay strong enough to saturate the material, and the heat has to go somewhere. That is why resolution is a manufacturing capability rather than a setting on a screen.
Why the material matters here
The pulse must exceed what the material needs to be fully magnetised. High-coercivity materials hold their pattern extremely well afterwards — which is the same property that makes them require a stronger pulse to write in the first place.
Then it gets checked
A written pattern is verified, not assumed. Magnetic viewing film makes the poles visible so somebody can confirm the part came out the way it was designed.