Finding the 28 digit key code number on my system devices ?

Go to Settings > [name on top] > Password & Security > Account Recovery. Under Recovery Key, if it is On, then your account is using a Recovery Key. The actual key is not stored on your device anywhere and it is up to you to remember it.

Set up a recovery key for your Apple ID - Apple Support

AND … if you are using a recovery key but don’t know what it is … you can generate another one.

HOWEVER

Use of a recovery key is entirely optional and is NOT recommended for most users.

For most users, it’s best kept “off” so as to keep Apple’s normal account recovery mechanisms available.