I know that the 737NG is not fly-by-wire, but are the control surfaces controlled with hydraulic systems that transfer the input from the pilot to the control surfaces via fluid (hydraulics), or is the pilots' input transferred to the control surfaces' electronic actuators via electric signals?
For the B737NG, the pilot stick inputs are transferred to hydraulic actuators, via mechanical (steel) cables. For example for the Power Control Units of the ailerons, as also depicted in this question:
The site where the image came from also lists a schematic pic for a fly-by-wire setup. There is still a hydraulic actuator but no more steel cable loop to the extremes of the aeroplane, only an electrical or data wire that sends a signal to an electronic controller near the control surface. The controller measures the surface position, subtracts that from the pilot input position, and deflects the hydraulic valve until the actual surface position = required position.
Of course it's hydraulic.
Electric actuators or power-by-wire is actually a far more exotic technology than fly-by-wire, and the latter is a hard prerequisite for the former.
Normal fly-by-wire is still based on hydraulic systems, with the electrical FBW system controlling the valves. And even in power-by-wire systems, the final actuator is typically a small enclosed hydraulic unit.
The newer 737 MAX has a couple FBW elements (spoilers), but is still a manual system, well short of FBW or PBW.