What is joule heating effect ?

Typically it is voltage across our heating element which we can control...not current. Current is much more difficult to prescribe, it requires intense design with transistors to make a constant current source.

Your formula to which to pay attention is P = V^2/R, where P is power dissipation, V is voltage drop across the heating element, and R is the resistance of the heating element.

Should voltage remain constant, the higher the resistance, the less power dissipation.

Specific resistance, more commonly called resistivity, is nothing more than resistance modified such that it is material dependent and not geometry dependent. We choose a metal of high resistivity, because this way we can keep the heating element wire short, and use less material, and still make a resistor which dissipates its heat at safe temperatures.

If we wanted really high resistivity, we could choose something like rubber. Would you make a heating element from rubber? No, it would be like not even having a component there at all, and no path for current to flow.

Why don't we use copper? If we design a geometrically identical copper heating element, it would have much lower resistance, which means much more power dissipation. And that excessive power dissipation can lead to melting of the wires and also demanding unsafe amounts of current from the master source of voltage.

Oxidation is a failure mode to avoid. It is a chemical change whereby the material corrodes with surrounding air, and higher temperatures cause the chemical change to happen more quickly. Therefore, to make anything which will operate hot, make sure it will not oxidize.