Power supply self control using transistor

I searched Google for RS Flip Flop and found something about logic gates etc. I don't see how it can be used as a transistor circuit alternative. I'm a slow learner. Can you please tell me how RS Flip Flop can be used as a transistor alternative.

And will a BC547 be enough to handle the current that arduino consumes or would I need a metal packaged transistor or a Mosfet?

Thanks in advance.