PhysicsOpenLab publishes worked experiments for people doing physics outside an institution — detectors, spectrometers, electronics. Their hydrogen line project documents a 21 cm receiver end to end.
Hydrogen Line Project Documentation (PDF)
It is worth reading next to the DSPIRA build for the parts where it makes different choices. Two people solving the same problem — get 1420 MHz out of the sky and into a computer — do not converge on the same hardware, and the places they diverge are usually the places where the constraint was cost, tooling or what happened to be in a drawer.