A common use case is around utilizing on premises SQL Databases for Cloud Analytics. There's a couple of options here.

Leave the sources on premises

Create Linked External Data Sources for BigQuery. Use BigQuery as the query engine.

Migrate your data to Cloud SQL or

Migrate your data to Cloud Spanner on GCP

For a multi regional solution, Cloud Spanner is the best bet. Migrate your SQL data to Spanner and then use ETL (DataFlow preferably) to pull data into BigQuery.