Davis,
if you're studying this subject, you really do need to get some books on it.
FADEC is electronic, as all ready said, and a computer works everything out using programme logic and laws and keeps the engine at its optimum speed for the conditions it is monitoring. I'm talking helicopters now, but if there are two engines....each one has its own FADEC and the two 'talk' to each other to keep the two engines 'matched'.
A mechanical FCU and its associated fuel governor needs to be set up for the engine it's fitted to, by the means of control link rigging and adjustment screws which increase/ decrease certain jet orifice gaps and/or increase/decrease spring load on internal levers and mechanisms.....similar to a carburettor on a (older type) car engine. The engine(s) is(are) trimmed in flight by mechanical pilot control to the governor.
By the way........ you're welcome.