How can you write a program that is long~~

I mean millions of lines, eh?  Does this actually happen in real programming, or is it just some enterprise thing where most of the programmers only know about selecting text, clicking copy, clicking somewhere else, clicking paste.  (the senior developers might know the arcane keyboard shortcuts, but they keep them secret as they are paid by the line of code)

Please explain to me like I am a three year old flatchested loli.