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?
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.