MacBook Pro 2012 record audio from phone through aux in jack

Solution 1:

The 2012 has only a mono mic input - this question explains the pin-out. MacBook Pro (mid-2012) Microphone Input

If you want stereo audio from Spotify, then you're really going to either need a USB 'sound card' of some sort - you can get something half decent for under $£€ 20, but don't go for the ones at $5, they're not worth it.

Alternatively, record from Spotify on the Mac itself. This will be a whole lot cleaner signal. It has traditionally been the territory of Audio Hijack which still remains my go-to for this type of task, but isn't cheap. There is a list of alternative audio routing apps/utilities on Capture audio correctly while capturing video calls on MacBook with earbuds