How can I get my iPhone recovery key?

If you have not previously requested a recovery key, you don't have one.

One also can’t be (re)generated for you. Not until you have a trusted device.

How to use account recovery when you can’t reset your Apple ID password - Apple Support

If you have an app downloaded by and associated with a different Apple ID, delete the app and re-download it,using the Apple ID you want to use.

Setting up a Recovery Key is an optional security measure that you would have had to set up before. If you do set this up, you will either need this recovery key or a trusted device to be able to access your account. If you did not write down the recovery key and don't have a trusted device, then you will permanently be locked out of your account. Below is the Support Article explaining the procedure and the relevant text is here:

This gives you more control of your account recovery methods and can help prevent an attacker from gaining access to and taking control of your account. However, if you lose your recovery key and can’t access one of your trusted devices, you'll be locked out of your account permanently.

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