What devices use rf?

Electronic devices as mostly used because of numerous advantage:It is cheap.It has high LifetimeIt makes the equipment light in weight.It consumes less electricity.

Any metal . Any metal with a good conductivity can be used; normally thin copper or mild steel that is tin plated is used the later is less expensive.

Yes you can, in fact I almost guaruntee you did to write the question - the RAM in your computer is likely DRAM, or dynamic random access memory, which uses capacitors to store bits. The problem with...

Solenoid valves, magnetic contactors, motors.

HydroElectric generators (think of Hoover Dam) , water wheel , steam engines by heating the water ,