Related:

https://www.youtube.com/results?search_query=simple+SDR

https://www.youtube.com/results?search_query=homemade+SDR

Paper: Tayloe Quadrature Detector, AKA "Tayloe Zero IF Quadrature Product Detector and Preamplifier":

https://www.norcalqrp.org/files/Tayloe_mixer_x3a.pdf

GitHub project (looks very cool!): "Quadrature sampling down converter software defined radio":

https://github.com/Dizzerin/Software-Defined-Radio