I want to sniff and decrypt HTTPS traffic of apps that ignore system proxy setting on macOS. If I understand correctly, common tools like Charles cannot help. How could I achieve that?
If you can install an extra CA, it shouldn't be a problem. Mitmproxy has documentation on how to set up transparent intercepting.
Get mitmproxy running. Set up that as the proxy for HTTP and HTTPS. Then load a web browser and to go http://mitm.it and download the newly generated certificate. Double click the file you just downloaded. Then within Keychains you'll want to tell the system to trust that cert ("always trust"). Then what I typically do is set the Router to the IP of the host running mitmproxy. Run mitmproxy with the transparent settings (-T) I believe.