Each thread starts where the last one ended. From that nail the loom scores every other nail by the average darkness still left along the straight line between them, chooses the best, and subtracts what the thread will cover. It is greedy and simple, and after a few thousand lines a face appears where there is only geometry.
String art is a lesson in perception: no line is curved, no line is grey, yet the eye integrates density into shade and edges into contour. The overlapping threads throw fine shadows onto the board, and that shadow is part of the picture.
The nail count and thread sequence are exactly what you would need to build this piece on a real board. Two hundred nails on a 60 cm hoop and three kilometres of thread is a weekend well spent.