What is the basic idea?

First of all, you need to find out what the hell this is all about... what the basic idea is here... and not just in this electrical implementation. Because it is a great idea that we can see all around us and we ourselves constantly realize it in various life situations. That is why I prefer to call it by such figurative names as "principle of life" and "active copying" rather than by the generally accepted "negative feedback".

The basic idea

The problem solved by this idea is simple - to make a quantity Y (Vout here) equal to another quantity X (Vin)... but it is realized in a complex way:

First Y is produced by another (different from the input) source (Vcc), then it is compared by subtraction with X and finally changed to the correct direction until Y = X (Vout = Vin) is reached.

Voltage follower

In the circuit of an op-amp follower, Vout is produced by a properly supplied op-amp, and the two voltages Vout and Vin are subtracted by contrary connecting them in series. The result of this subtraction is a "floating" voltage Vout -Vin that requires an op-amp with a differential input.

It is interesting that this idea was invented and implemented in electrical form a long time ago - in the 19th century. This made it possible to make an "ideal" voltmeter without any electronic gadgets such as transistors and operational amplifiers. In my story "Ideal" voltmeter (for now, in Bulgarian), I have told in detail how together with my students I reproduced this famous experiment in the laboratory in order to show the power of this idea. Here are three pictures from the story:

19th century electrical follower ('ideal woltmeter')

21st century op-amp follower

21st century op-amp follower - conventional circuit diagram

Voltage inverter

In contrast, in the circuit of an op-amp inverter, the two voltages are subtracted by 2-resistor summing circuit. Its output voltage is referenced to ground; so the op-amp input can be single-ended.

Generalization

The benefit of all this is that the load (next stage) will consume current from Vcc and not from Vin… ie. Vout is a "powerful copy" of Vin.