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TRF sets are basically an LC filter tune-able over the AM band followed by a stage of amplification (rinse and repeat a stage or three), a grid-leak detector (google for more on that I forgotten how those work), and an audio amp stage or two.
Regenerative sets are a complex way of making minimal tube count work like a big TRF....Basically the RF is fed back in a positive feedback loop to J-U-S-T shy of the point where it oscillates (you'll know when it does)...IIRC some managed detector/audio gain function from the same single tube.
There were also other schemes like one where there was a single detector tube and the TRF stages did double duty as audio amps.
There were a few more designs, but my knowledge is rather limited. 20's sets have always been stupid expensive, hard to get parts for, and hard to power putting them out of my reach. I've owned/worked on 3 early AC powered TRF sets, and just got my first restorable bat set a week ago.
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