The iOS operating system is very secure out of the box. Apple carefully evaluates and only allows to the app store apps that are deemed to function within Apple guidelines.
Even though the OS is secure, there are additional settings you can change to improve security.
Most importantly, keep iPhone updated to the current version of iOS as each release big or small, generally fixes multiple security vulnerabilities.
– All iPhones (all smartphones generally also) should have a passcode setup. If Touch ID or Face ID is available, utilise this if you can.
– As iPhone has a camera, a microphone and can track your location, only allow these functions to work within specific applications for a reason.
In Settings, Privacy, you can check Location Services, Microphone, Camera options to only allow apps who need this to have access, worth a quick check periodically.
– The Bluetooth and AirDrop technologies should be turned off if not needed, ideally, AirDrop should be limited to contacts only if needed.