Jim aren't you an RF guy? HAM/CB?

Yep

Dont they just use and inductor and capacitor and resonate at the FR?

Bingo. Geek-speak for it is a "tank circuit" and when tuned to the desired harmonic, it accepts that frequency and rejects others. Back in the '20s and '30s a bunch of people came up with different circuits to do the same thing (Hartley/Colpitts/Armstrong oscillators, etc, etc) but they're all just variations on the same LC circuit theme with different numbers of caps or inductors.

Cool stuff. I thought about getting into Ham but I can't afford another hobby

Yes everything you just said about class C amps just came back to me. lol

The general car audio term "high current amp" has nothing to do with the amount of current that the amp draws. It was used back in the day to describe the competition style amps that were stable into low impedances. There are two ways to make power, one is to have high voltage on the outputs into a relatively high impedance, the other is to have fairly low voltage but relatively high current by wiring the amp into a low impedance. The amps designed to make power via the second method were called "high current." The power supplies were overbuilt compared to a similar 2 ohm stable amp with the same 4 ohm power rating. The output devices were oversized and there were typically more of them. The amps didn't have to be all that powerful or draw a lot of current. U.S. Amps made one rated at 12.5W per channel and it was quite small. At full tilt into a 0.5 ohm load the amp only drew on the order of 30A but it was a high current design. PPI made a similarly small high current amp in the ProMOS 12.