What formula is used when applying layers in normal mode?

Question

Algorithms

I understand that the overlay occurs pixel by pixel (how else?).
Then let the first pixel be RGBA(r1, g1, b1, a1) and the second pixel be RGBA(r2, g2, b2, a2).
What will be the final pixel?
Or is it better to count in other units altogether?

Answer 1

Here it is described in detail:
https://habr.com/ru/post/328386/
You can also see how the BitBlt / StretchBlt function works in WinApi, there is a dwRop parameter
Or search for Alpha Blending a Bitmap