How exactly does fragment shader work for texturing?

Question

I am learning opengl and I thought I pretty much understand fragment shader. My intuition is that fragment shader gets applied once to every pixel but recently when working

Answer 1

first : in core context you still can use gl_FragColor.

second : you have texel ,fragment and real_monitor_pixel.These are different things. say this line is about convert texel to fragment(or to pixel idk exactly what it does):

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);

when texel is less then fragment(pixel)