How to realize a negative voltage with a positive regulated DC output?
Following doesn't answer how to connect your converters to get -12 V.

I would not be able to create a -12V by just using the same technique; a buck for 32V and connect the terminals in reverse for -12V. I didn't get why this is true.

Confusion arises since you have assumed that the 12 V converters are devices which have only two terminals. They actually have 4 terminals. Two of those terminals (reference or 0V terminals) may be shorted inside the converter (left side picture). See the diagram below. If you connect the 12 V of one such converter and 0 V of another together, you are effectively shorting the 12 V and 0 V of the same converter together.
If the input and output of the converter are isolated from each other, this type of short doesn't occur (right side picture). A common way the input and output terminals are isolated is via transformers. There is (usually) no electrical connection between the primary side and secondary side; the connection is magnetic.
See the diagram below.