wallet – Get my private key

My goal is to understand. I don't want to make money with mining.
(I have a server without GPU)

This is what I did:

I have downloaded, created and installed the github bitcoin source code.

It seems to work because I can interact with bitcoind with the bitcoin-cli command.

I know this is not a good practice, but I want to "see" my private key.

I have read several documents and tutorials and they said that I first need to generate an address, then call the dumpprivkey method.

I like this:

$ bitcoin-cli getnewaddress
xxxxxxx

$ bitcoin-cli dumpprivkey xxxxxxx

I don't understand, but if I repeat this operation twice, I get 2 different private keys.

So this is not the private key I am looking for … What are those private keys?

Thank you