I have a Yamaha P45 whose audio I want to record on my Macbook Pro. I followed instructions from this video so to summarize:
I have a 3.5mm stereo y-splitter plugged into the Yamaha P45, and two patch cables running from the y-splitter: one into my Audio-Technica headphones, and the other into a StarTech TRRS adapter that I have plugged into my Macbook Pro. When I play keys on the piano, I can hear them in my headphones so the y-splitter probably isn't the issue.
The only input audio I see listed when I try recording with any software is the built-in microphone (internal), so the keyboard's audio isn't getting picked up by the Macbook Pro.
As an aside, I do notice that if I plug my Apple headphones into the headphones port of the y-splitter I can receive audio output from the Macbook Pro.... so does that mean the y-splitter should be able to capture the keyboard's input or that it actually is capturing the keyboard's input but the Macbook Pro just isn't recognizing it?