Where is the password for iPhone backup stored?

The password is not stored on any of your devices, so hopefully, you will remember the password or find the piece of paper where you wrote down this very important information.

You will not be able to access the data in the backup without the correct password.

A password as important as this, should have been written down somewhere for future recall. You CANNOT find it anywhere. It's encrypted. You can guess as many times as you want, but until you remember the correct encryption password, the backup will remain locked. There is NO workaround for this.