Some basic computer audio questions

Hi mcrib
My experience
1) Used USB out to an external dac— SQ not just "better" but in another dimension altogether2) No idea, but you could have one made inexpensively using aftermarket wire 3) No idea:) I doubt the difference (if any) is significant, compared to possible improvements elsewhere in the chain!
Regards

I have a late 2015 iMac and the headphone jack doubles as a mini-optical digital output. Not sure but I would guess it's the same for your Mac since I think all iMacs had that feature during that time period.

For analog output you use a stereo-mini cable but for digital output you use a mini-Toslink to Toslink cable or a Toslink cable with a Toslink to Toslink-mini adapter.

The optical cable can go directly to a DAC with Toslink input. I don't know if that's preferable to USB output but it works extremely well for me. It's not necessary in using the optical output but I have a Wyred 4 Sound Remedy Reclocker between the iMac and a Pro-Ject Pre Box S2 Digital DAC to reduce the jitter that's inherent in most optical outputs. Using the digital output will certainly give you much better audio than the analog output.

thanks for the responses!