How does a Proxy contract communicate with the underlying Implementation?

Question

When a user interacts with a proxy contract, they send a transaction signed by themselves to that proxy. My question is, how is that transaction communicated to the underlying implementation contract? Does the proxy send another transaction to it, and if yes, who is that signed by? Or is it a relationship more like the proxy contract inheriting from the implementation?

If you could also provide the source of your answer it would be very appreciated, thanks for any input.

Answer 1

Smart contracts call each other by address. It is sometimes referred as "internal transaction" but this is incorrect terminology.

Here is OpenZeppelin proxy implementation.