Why does my phone want my passcode after a restart?
Because your data is encrypted, and the keys that unlock it are tied to your passcode. After a restart the phone can't unlock most of your data on its own, so fingerprint and face unlock won't work until you enter the passcode once.
Right after a restart a phone is in a state called BFU, Before First Unlock. Only a few keys are available, enough to show the lock screen and take a call. Your photos, messages and most app data stay encrypted.
Your first passcode entry unlocks those keys and keeps them in memory until the next restart. That's also why newer iPhones and Android phones restart themselves after about three days without being unlocked, so a stolen phone falls back into the more protected state.
- Use a strong passcode, since it protects the encryption keys
- Restarting a phone you think is at risk puts it back in the more protected state
- Biometrics are a convenience on top of the passcode, not a replacement
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Storage & Memory
From the Inside a Phone course. RAM vs flash, and why storage is encrypted.