How can I find the recovery key for my MacBook Air?

see if you can find it here

1. Choose Apple  menu > System Settings, then click your name.
2. Click Sign-in & Security.
3. Click Account Recovery.
4. Next to Recovery Key, click Manage, then enter your Apple ID password.
5. Click Turn On, click Use Recovery Key, then enter the password for your Mac.

if you cannot find it, it means that you have already set up a recovery key, and that key is meant to be stored somewhere safe