It’s been a looong time since I made a Heathkit (I think the last one was an oscilloscope in the 80s) but I do remember a theory of operation section in the manual, separate from the very clear assembly instructions. It’s true of the ones I randomly checked in the archive [0]. That said, the mail-in repair service for when things go sideways was a fantastic feature that not many kit companies had/have.
The first H-151 I built had a flaw. We went to the Heathkit store, about 45 minutes away, which provided the instructions for the fix. It was something like "scratch this trace off the circuit board and solder a wire between these two points."
[0] https://www.vintage-radio.info/heathkit