I made a working model in LTspice of 2 computer SMPS's I took apart, and what sort of load should I add to the voltage rails to simulate a basic computer ? If I was to add a res, cap, inductor, what configuration and values would they be for the few hundred Watts a PC uses on average ?

 

A resistor to ground with good bypassing should be sufficient unless you are looking for some particular phenomenon.

 

well what do billions of transistors tuning on off look like, and all the various switching step down regulators look like ? I think I heard it looks more capactitive, but IDK enough

 

The load for simulation depends on what you are trying to simulate.

A real computer could be a simple load or not so simple.

a) Sudden power level changes

Processors are CMOS these days and use minimal power when not running
but large amounts when running. So a processor might go from a few
watts to 100 watts in a few processor cycles (at 3 GHz that's less than
a nanosecond). You won't see the transient that quickly since there
is another switcher between your output and the CPU (12 volts to 1
volt?) but you will see it over longer time periods (100 of uS? mS?).
This isn't just a one time change, the power load could bounce up and
down in a pattern or at random depending on the computation.

b) Negative resistance load

The load is the input to a switching power supply and with a constant
load that supply will pull less current at higher voltage (constant
power). So if your supply outputs a higher voltage at lower current
it might cause the load to draw even lower current causing higher
output voltage into the switching supply etc. This might
stop changing at some point or it might oscillate up and down...