VB-Cable from VB-Audio Software for piping audio from SDR# into WSJT-X.
I mostly use macOS and Linux but for some reason SDR# seems to be the SDR software I like the most and it only runs on Windows.
A 2012 MacBook Pro is used to dual boot Windows for running this stuff as you see above right.
Today I noticed that there is a version for macOS and so far it looks good. (The software is free but "donation ware" and I've send them US$10 in appreciation of the macOS version).
A new device simply appears as an input and output device and settings are available in the Audio MIDI control panel.
I'm aware of Rogue Amoeba's Loopback but it is US$109 and seems more complex than is needed for this basic task. Their other software is excellent though.