Origin of the screen coordinate system

I would like to understand the screen coordinate system used in three.js. Specifically, is the origin at the upper left corner?

You mean coordinates in postprocessing shaders, in material shaders, or overall - in rendering of the scene in 3D?

I mean the frame buffer that is used in normal rendering. On this, we can define mouse locations in (x,y) terms. Where is the origin of this (x,y) frame? And also, do the coordinates in each direction start with 0 or 1?