How focusing screen works?

[tecboy]

I've never understood how focusing screen works, and how the auto focus points appear inside the viewfinder? What material does it made of?

 

[astroNikon]

I'm not an expert at all about this but this article might explain it better
==> https://www.learn.usa.canon.com/app/pdfs/quickguides/CDLC_FocusingScreens_QuickGuide.pdf

 

[dxqcanada]

How are the red focus point indicators displayed on a dSLR's focusing screen?