Apple believes in your right to privacy. Here is some advice on how to use the tools it has given you to protect your privacy on an iOS device.
You probably already use a 4-digit passcode, but you can improve that with a 6-digit or alphanumeric code.
You change this in Settings>Touch ID/Face ID & Passcode, select Change Passcode and then tap the small Passcode Options dialog. Alphanumeric codes are harder to decipher, just make sure you remember the code.
