master git repo or website to search for inspiration when you need to design a thing. That is great example: https://github.com/arturo182/pico-compatible-boards
To check If you can find a reference design for a chip, you should check manufacturer website first to see If they have reference design. They generally do. After that,
One of the good example is OSHWA https://oshwlab.com/ from EasyEda https://workspace.circuitmaker.com/ has nice examples. Under the flag of Altium And you can check GitHub of course. www.github.com
To check If you can find an example board or module in maker level world: Adafruit Sparkfun Seeed Studio Waveshare