Not in any detail, but as a hand-waving explanation it keeps tracks of the Sun and the star Canopus, so by two fixed reference points you can have a known orientation.

I can't even begin to imagine how you would go about building an automated star tracker in the 60s.

The Sun is the brightest star and Canopus is the third brightest star (Sirius presumably is not in an appropriate position to be detected), so you don't really need a proper star tracker, you just need a brightness sensor.

But the ICBMs did. They also had star trackers to help them navigate, and they needed more robust maps than just the Sun and Canopus, since they had to be able to fly 24hrs a day, 365 days a year. Different problem space.

There's a bunch of publicly available documentation about how the Canopus star tracker on the Voyager probes works out there, last I looked, and it's quite an interesting design by modern standards. It uses an image dissector tube, which is weird and long obsolete vacuum tube tech that can measure the light in an electronically-controlled section of an image, to scan a slice of the sky around the roll axis of the spacecraft looking for an area in the right intensity range (which is fairly easy for Canopus since it's generally the brightest thing in that part of the spacecraft's view so long as the roll axis is correctly aimed at the Sun), and there's a bunch of hardwired digital electronics to control it and use that to adjust the spacecraft orientation.