Isolation Circuit Question - How does this feedback work?
The thing is, I have no idea how this thing works

The whole circuit is a flyback converter so there's your first term to start googling.

But, I have no idea how the jeezless thing is getting proper feedback
through the optoisolator

The TL431 acts like a voltage comparator - as soon as the &quot;right&quot; output voltage appears on the isolated output, it turns the UC3842 off (via the opto-isolator) and the DC output voltage drops a tad (under load) and then (microseconds later) the TL431 signals to the UC3842 that there is an under-voltage situation. This type of feedback can be a little unstable and can lead to a bang-bang control of a sort.
The threshold at which the &quot;regulation&quot; occurs is when the junction of R66 and R67 is 2.5 volts (that's the reference voltage inside the TL431). If you do the math, that allows 1.042 mA through R67 and R66 - this means that the output voltage regulation point is 21.25 volts nominally.

I have no idea how the Isense pin is being used here.

Because a flyback converter circuit stores magnetic energy in the 1st half of the cycle without reference to the output winding, you have to use a current limiter to protect the transformer from core saturation - that is what Isense is being used for. A fixed amount of energy is stored in the primary winding and, in the 2nd half of the cycle, that energy is released from the secondary, via D2 into the output reservoir capacitor that appears to be missing in your circuit.