Yeah, I was going to link the exact thing @TomWhitwell did - I was looking at it only the other day (in the Rings) circuit.
What you’re seeing is, I believe but might be wrong and an idiot, using an opamp as a differential amplifier: the combination of a precise choice of resistors along with the -10V reference means that a single opamp both offsets and scales your voltage. The icing on the cake is that the 3.3V opamp limits the maximum voltage going into the Teensy.
If you have a look at the schematics for Rings, you’ll see that the modulation inputs are scaled from -8/+8, and the v/oct scales from -1.5-+5. Both scale down to a 0-3.3 range, though note that sometimes the scaling means the minimum possible voltage will be > 0V. Circuit simulators are a good way of testing the output range - stick a 16V p-p AC sine wave in at one side, and watch what comes out the other.