This is the new Stereo Line-In… and Stereo Line-Out.
That’s a Behringer UCA-202. I have two. Also see: Behringer UCA-222 and UFO-202 which has a a built-in phono preamp for conventional analog turntables.
My so-new-it-squeeks Air still has a conventional USB connection. Actually two, in addition to the Thunberbolt I use with various adapters for wired network and backup hard drives.
There are certainly other ways to do this, but that’s been my hero device for a while now. Also see:
https://forum.audacityteam.org/t/sound-card-reviews/8375/1
Be careful not to fall for one of the ultra-cheap USB adapters such as the ICUSBAUDIO by Startech.
https://www.startech.com/Cards-Adapters/Sound/USB-Audio/USB-20-to-Audio-Adapter~ICUSBAUDIO
Turn a USB port into a Stereo Sound Card. Sounds terrific, doesn’t it? It doesn’t. It has a stereo headphone connection and a mono microphone connection. It’s not stereo Line-In. I use one of these for a microphone connection for minor recording jobs that don’t need a full studio.
The UCA202 is certified for overdubbing/sound-on-sound because of its built-in zero-latency monitoring. No I don’t recommend using earbuds for advanced musical production. Those were graphically convenient for the photo.
Fair warning, the combination of El Capitan (10.11.5) and Audacity 2.1.2 can give stuttering or damaged recordings. Test it thoroughly before you commit to a critical recording. The days of pressing Record and walking away vanished with the Mac Stereo Line-In port.
Koz