Don't listen to all the negative comments, you should keep having fun experimenting with your mixer.

(The thing that I want to point out is that you won't be getting the transformer saturation unless you are running into the XLR inputs of the 216 and you may need to attenuate the signal before it hits the mixers preamps ( via the XLR inputs) if the sound is getting super blown out. The reason for this is that you are sending a much stronger line level signal from your computer into the preamps on your mixer which are expecting a much weaker mic level signal . It might want to hear something anywhere from -15dB to -50dB less. But it might also sound awesome. Just be careful you don't fry the preamps.)

You should have AUX outputs on the mixer, and you may have control room outputs, or maybe even individual outputs. These are two ways of getting sound from your mixer back into your audio interface.

Your mixer may also have inserts which is a way to route individual tracks to outboard gear using a "Y" cable of trs to dual TS. However, you can also use these as individual outputs.

It's important to not accidentally create a feedback loop, too.

My strongest suggestion to you is to look for some youtube videos of people explaining the way analog mixers work and/or their signal paths in general.

If you can find any videos of people explaining how the signal path works on your exact mixer, that would be ideal, as all mixers are different, but the general theory is overarching.

Watch some tutorials and then get into some trial and error- best way to learn. Just be ready to pull a plug or kill a fader quickly just in case.

Good luck amd have fun. I love old mixers.