you place a Switch it automatically has a circuit to select which power it will connect?

No it is a series of over 20 custom accumulators that act as power limited cables
lets say you have 2 power grids A and B. B needs power say 100mw and A can can provide 50-60MW depending on what is running in it

if you would connect the 2 with a cable then you would brown out and that will lead to a black out so not ideal.
IF you connect A and B with my accumulators ( useing the appropriate sized ones) A will only be able to send the excess power of 50-60MW to B so there will be no brownout in A and B will run more or less with that extra

So is it a circuit network connected bunch of Combinators to Limit the power Draw/drain from either Source A or B power source does that mean 2 Different Lines of power source connected to the Perimeter of the Defenses?

no it is not circuitry it uses base game mechanics to achieve that , it is a box , no circuitry knowledge needed to use them

does that mean 2 Different Lines of power source connected to the Perimeter of the Defenses?
NO
example:
A: power plant + accumulators + defense perimeter
B: everything else

A is what u deem absolutely necesary to run no matter what , B is the rest