what is the difference between external and internal circuit?
Do you mean what is the English meaning of these words?
As an example, you might have an integrated circuit called a 555.
This has a lot of components inside it and it needs a few extra components to make it work as an oscillator.
The ones inside it cannot be changed and these are called the internal circuit.
The ones outside the chip are ones that you supply and these can be changed to get different frequencies. This collection of components and how they are connected is called the external circuit.
Separate names with a comma.