To lock apps on a mobile phone, you can use built-in security features or third-party app lockers. On Android devices from companies like Samsung or Xiaomi, you can go to Settings, then Security or Privacy, and enable the App Lock feature to protect selected apps with a PIN, pattern, or fingerprint. On iPhones made by Apple, you can use Screen Time to set app limits or enable Face ID or Touch ID for certain apps that support it. This helps keep personal information secure from unauthorized access.