I think you may be confusing sessions with "resources"? https://boto3.amazonaws.com/v1/documentation/api/latest/guide/resources.html

Sessions handle the configuration, your credentials and the AWS region. They are not specific to a service; to use a service you create a client or resource for that service using the session. Every service client in boto3 has a parent session (though as noted in the article, you may be creating your clients/resources on the default session). "Resources" provide higher-level abstractions on top of the clients, for services like DynamoDB, S3, SQS, etc. but not, for example, Secrets Manager.