How to fetch Amazon Cognito Identity ID (user_identity_id) for the user from the lambda function?

In the Amplify documentation, under the Storage/File access levels section there is a paragraph that states:

Files are stored under private/{user_identity_id}/ where the user_identity_id corresponds to the unique Amazon Cognito Identity ID for that user.

How to fetch user_identity_id from the lambda function?

Request to the lambda is authorized, the event.requestContext.authorizer.claims object is available, I can see the user data, but not the user_identity_id.