Linear voltage regulators, such as the ones on your bench are made of circuits like this one. They also include the voltage reference generator. The “pass transistor” Q
4 is of sufficient rating for the maximum output current(You may be able to see large pass transistors mounted on heatsinks on the backside of some of the power supplies in the lab). Multiple buffer stages may be required(such as Q
3) to drive the base current of Q
4. Usually the feedback loops have more stages for more gain. For tracking dual power supplies, there is effectively another feedback circuit that looks like an inverting amplifier with the positive V
out as the input. On modern integrated circuits housing entire systems, like large portions of a radio, it is common to find even upto a dozen LDOs powering various blocks.
LM2940 is an example of a commercially available LDO. Page 12 of the datasheet has the schematic diagram.