Phil has it just about nailed down.
Another way to see the same thing is that we don't run our engines to the RPM they could ultimately reach on whatever venturii ID we have. We restrict RPM by prop, fuel, plug, needle, to get the airspeed and running characteristics we want.
The prop is the load... If we went to the ultimate RPM possible on a given prop, that would be more than the RPM that "behaves" dependably for CLPA.
Fuel influences how the combustion proceeds: Alcohol is good at cooling when it vaporizes, so it keeps the engine from overheating if we do it right. An excess of alcohol somewhat damps the combustion by chilling it, as does the oil. We don't fly stunt with the engine totally leaned out to were we couldn't dial in a few RPM. So, there's always a bit of excess fuel going through the engine...
Plugs operate in some magical, mystical way, and we need to start by trying what others have found works in our particular engine. If that doesn't work, try other well-recommended plugs until you find what does.
What the needle does is pretty well suggested above. It is nice to have a progressive needle - which means that the engine richens or leans consistently and steadily as we adjust.