Where do you get the "reference voltage" for the voltage comparator and opamp? Or which one is the reference voltage? For example, the reference voltage of the 555 timing block

1) What is meant by reference voltage. What is it used for.

A reference voltage is a known, precise voltage. You can use it to compare and analyze it with other voltages.

For example, the 555 has three 5k resistors to divide the voltage. But its reference voltage is fixed.

②Have you looked at the op-amp schematic, it has +- 2 icons. You can define 1 of them as the reference voltage. Then the other 1 is the reference voltage.

When feedback is introduced then the reference voltage of the op-amp will change to stabilize the circuit. So it's not like there is no reference.