Where is my 28 character recovery key?

Wherever you wrote it down and saved it or in whatever password app you use to keep things like that. For security, the recovery key is not stored anywhere automatically for you. It is up to you to keep it safe and available.

You can write it down and store the paper somewhere secure. You can keep it in a password manager, or a secure locked note. But it is up to you to keep it for when you need it.

See -> How to generate a recovery key - Apple Support

”Using a recovery key is more secure, but it means that you’re responsible for maintaining access to your trusted devices and your recovery key. If you lose both of these items, you could be locked out of your account permanently. With that in mind, it's important to keep your recovery key in a safe place. You might want to give a copy of your recovery key to a family member, or keep copies in more than one place. That way you always have your recovery key when you need it.”