Security is a big deal on phones because they contain so much private information. The Samsung Galaxy S9 and S9+ includes Knox, encryption and a range of biometrics to facilitate unlocking.
Top security tip: Biometrics aren't foolproof, because when they fail your device reverts to PIN or password to unlock. Therefore, your device is only as secure as the password or PIN you use, as anyone trying to break into your phone can always opt to head straight to these unlock methods. Biometrics are there for convenience, not security.
Enable fingerprint/face/iris security: To use your fingerprint/face/iris to unlock, head into settings > lock screen and security > screen lock type. Here you can select the biometric you want. You'll have to set a back-up PIN or password at the same time to provide additional security. Top tip: if using fingerprints, then register fingers on each hand so you can unlock however you are holding your phone.
Enable Intelligent Scan: Intelligent Scan combines face and iris, so the phone can use either or to unlock the phone. This means you just have to look at the phone and it will unlock in most situations. You'll need to register both your face and your iris to make this work.
Automatically wipe your device: If you're worried about your phone falling into the wrong hands and being cracked, you can have it automatically wipe. Head into settings > lock screen and security > secure lock settings. Here you'll find the option to auto factory reset if 15 failed unlock attempts are made. Lock network and security functions: This option will mean that your network settings cannot be changed while your phone is locked. This makes it easier to locate your phone if it's stolen. However, it also means you have to unlock your phone to engage flight mode. Head into settings > lock screen and security > secure lock settings to find the option to turn it on or off.
Unlock with the on-screen home button: Rather than hitting the standby button or the fingerprint scanner, you can long press on the on-screen home button to unlock. If you have security, it will go straight to unlocking, for example to the iris scanner or PIN code entry. If you have no security it will just unlock your phone. Head into settings > display > navigation bar and select "unlock with home button".
As you've written, biometrics is not foolproof and a dead owner's fingerprint biometric can be used to unlock a phone; leave aside the other biometric options.
