API Gatewayresource policiesare the native way to control who can invoke a REST API, including allowing access from specified AWS accounts. AWS documentation says resource policies can be attached to an API to control whether specified principals can invoke it, and it explicitly lists users from a specified AWS account as a supported use case. Private APIs are callable through interface VPC endpoints, but the primary authorization control for which external accounts may invoke the API is the API’s resource policy. Endpoint policies can add controls on the VPC endpoint side, but the clean cross-account access control for the API itself is the resource policy.

============