In the Cloud SQL for PostgreSQL documentation are some articles related to importing/exporting data.
Regarding the Best practices for importing and exporting data, here are some of them:
Also known issues are referred to in this documentation.
As shown in this Stack Overflow answer, you can use pg_dump to extract the table. This page describes exporting and importing data into Cloud SQL instances using pg_dump and pg_restore.
The postgres_fdw is an extension allows tables from other ("foreign") PostgreSQL databases to be exposed as "foreign" tables in the current database. Currently, this extension works for two Cloud SQL private IP instances within the same VPC network, or for cross databases within the same instance.
To connect to databases within the same instance, you cannot set host to localhost or to 127.0.0.1. Instead, you must use the IP address shown for your instance in the Google Cloud console.
Additionally, you cannot choose, in the Google Cloud console, the Allow only SSL connections button for an instance where foreign data is stored. Only cloudsqlsuperuser can be the owner of a postgres_fdw foreign data wrapper.