Reference Material:

https://docs.aws.amazon.com/kms/latest/developerguide/key-policy-default.html
It's the first policy statement in the default key policy for KMS keys created in the AWS KMS console.
{
"Sid": "Enable IAM User Permissions",
"Effect": "Allow",
"Principal": {
"AWS": "arn:aws:iam::111122223333:root"
},
"Action": "kms:*",
"Resource": "*"
}
When the principal in a key policy statement is the account principal, the policy statement doesn't give any IAM principal permission to use the KMS key. Instead, it allows the account to use IAM policies to delegate the permissions specified in the policy statement. This default key policy statement allows the account to use IAM policies to delegate permission for all actions (kms:*) on the KMS key.