Look at the the project board here
A line is composed of a multitude of interconnected triangles, they are generated from "key point" which are the points having as coordinates the places where you have slipped your finger/pen and handled by your device. Several linked triangles can form a trapeze which allows to manage the force communicated with the apple pen in order to vary the width of the line on the same line.
Image from ios_metal_bezier_renderer
I use the Catmull Rom interpolation, it generates points between each key points
With interpolation:
Without interpolation (only the key points):
