What is the formula that is used by three.js to create the pattern that is seen when two overlapping 2-D surfaces are shown in SageMath?

I'm trying to understand the pattern (looks similar to an animal print) that shows when two different colored planes are plotted on (almost) the same plane. What is the formula that SageMath uses--using three.js--to create the pattern shown in the graph? The SageMath question/support area sent me to this support section for answers.

Example: Here one plane is slightly larger-- which makes SageMath show them both, but with a pattern. Also, as you move/manipulate the graph with the mouse, the pattern changes. What formula or information does SageMath (three.js) use to show the pattern?

I used the Sage Cell Server online to plot this (below) at https://sagecell.sagemath.org/:

sage: M = implicit_plot3d(lambda x,y,z: x, (-15,15), (-15,15), (-15,15), rgbcolor= (0.0, 1.0, 0.0), frame=true) sage: N = implicit_plot3d(lambda x,y,z: x, (-15,15), (-15,15), (-15,15.5), rgbcolor= (0.0, 0.0, 1.0), frame=true) sage: M+N

Thanks for any information you can provide!