You back up your wallet by going to File> Backup wallet. Then you will be asked where to store the backup. File wallet.dat will keep all your personal wallet information, including the unique Bitcoin address created automatically and the private key. Reusing an address after a transaction is not recommended as it is a security risk. Send the remaining data after the transaction to a new wallet, which is linked to a new private key. Personal locks are a detail in bitcoin. A private key is automatically associated with a bitcoin address and both are unique, created for you. No one can break or recreate it.
Bitcoin deals are always open and visible to everyone in the blockchain trade show website. Never store a private key or even import it into any device if you do not need it.