ISA is a set of average conditions based upon a nominal 45 degrees latitude around the vernal equinox, with some standard figure for humidity at sea level, and a tropopause at 11km. Virtually never will you ever actually see those conditions. It's based upon a set of balloon soundings and aircraft observations done by NACA and NASA between the 1920s and 1960s.
They're a hellishly useful baseline standard, because they provide something that we can reference everything else - everything is ISA +/- and people like me use standard ISA figures for designing and certifying aeroplanes.
The actual equations used for defining ISA are exponential equations with a lot of digits. However, below 10,000ft it looks remarkably like a straight line, and you can approximate it with 1.98deg/1000ft.
Generally speaking, it's based upon a stationary column of air at ISA S/L conditions at the bottom, with of course all that matter on top, so the pressure reduces with altitude. That of course affects the values.
Once upon a time I could derive all these equations from scratch with a pencil and paper. Please don't ask me to now!
G