Tonic Structural uses service accounts to access both the source and destination datasets. Even if the source and destination are in the same BigQuery project, Structural recommends that you create separate service accounts for the source and destination.
When you create the service account for the source dataset, you should use the "Viewer" role to grant the service account access to the project. Structural does not require write permissions to the source dataset.
When you create the service account for the destination dataset, you should use the "Editor" role to grant the service account access to the project.