how can I see a password stored in face id for an app?

FaceID doesn’t store passwords, it simply forwards a “pass / fail” to the app requesting a facial authentication.

Any app passwords which are stored will be in your keychain. (Settings -> Passwords)

Lastly, you can see which apps use FaceID in Settings -> FaceID & Passcode -> Other Apps.

That’s a very astute observation.

Short answer is that it’s up to the app developer.

Some apps cause your device to reach into keychain for your creds.

Other apps apparently simply ask your phone, “Is this the same guy who registered?” (and never look at keychain)

I have a small number of apps in this category which don’t even have entries in the keychain.

Further to this Q, I would like to know... If you can log in to an app using Face ID on iPhone, does Face ID process use the log in details stored in the keychain? Because, for one app in particular, I can log in with Face ID but there is no sign of any log in details in the keychain (which I'd like to access/see).

Great answer, thanks!