First of all, I advise you not to work with freelancers if privacy and reliability are of great importance to you. If you cooperate with a team of experienced developers like Arrible https://arrible.com/services/pc-game-development/ , providing full-cycle services for PC game development and more, there is no reason to doubt the confidentiality of the project. Besides, you sign NDA at the beginning of your work.

I've always wondered how confidential game development is, different projects and all that? How can I know for sure that the code of my future game won't be used elsewhere in other projects?

Really good stuff here.