If you are recovering your older MacBook’s password, you are being asked for the File Vault Recovery Key.
This is not the same as the more recently implemented Apple Account Recovery Key.
The recovery key for File Vault was displayed on-screen ONCE - and hopefully recorded and saved in a safe location - back when the brand new computer was being initially setup.
Hope that detail helps jog your memory as where it might be stored.
Unfortunately, your prospects to regain access to your data are fairly bleak if you can’t find it or remember the password.