This resembles the output stage of an op amp. It must solve a similar calculation. It forces the output to a certain voltage. It does this, whatever the resistance of the load, and whatever voltage is coming from the other end of the load.
The decision-making circuitry needs to be able to decide which transistor to turn on, and how much to turn it on. It is wasteful to have upper and lower transistors both on simultaneously (a condition known as 'shoot-through'). It's hard to figure out whether the decision is a function of voltage regulation, or current regulation.
Many internal schematics have a current mirror in one of the middle stages, which governs the output somehow.