Where is Face ID or fingerprint sign-in?
Face ID and fingerprint sign-in are made for a phone app, so they are not available in a web browser.
Where to find it
SOS has no App Store or Google Play app today. In a phone app, you would open Settings, then Security. Under Biometric sign-in, you switch on Face ID or Touch ID / Fingerprint.
The switch shows only if your phone supports it and you have set up Face ID or a fingerprint in your phone's own settings.
SOS asks for your password once and keeps it in your phone's secure storage.
On the website
The website does not have it. That includes SOS added to your phone's home screen. Security says "Biometric authentication is not available on this device." Sign in with your email and password.
What it does and does not do
Your fingerprint or face data never leaves your device.
It unlocks a saved sign-in when you sign in. It does not lock the app while you use it.
Signing out removes the saved sign-in, so you switch it on again afterwards.
If your password changed on another device, the saved sign-in is out of date. Sign in with your password, then switch it on again.