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. Different builders solve the same problem: getting 1420 MHz signals from the sky into a computer. Their hardware choices differ with cost, available tools, and spare parts.