Ethereum node - private key

Hello! Please help me with one question.

It is known from the documentation that only the "eth_sendRawTransaction" raw sending method is supported for sending. And this requires a private key (in the article we found how to send a raw transaction. In this article it is written about a private key that can be extracted from a file using a library on JS). But it isn't clear to us where to get the private keys of the addresses created on the node. And it isn't clear where you need to get a file that contains secret data.

And if there are examples in C#, I would be very grateful. Thank you!