You specify ownership of the work product in the contract. I believe the default in most jurisdictions is the client owns the work product (called work for hire

You specify ownership of the work product in the contract. I believe the default in most jurisdictions is the client owns the work product (called work for hire) but the contract should always spell that out to avoid any misunderstanding. You may have to consult an attorney to write the contract so you own the IP and grant a license to the client. The client may ask to pay for the license rather than pay for your time to write code they don’t own.