the base64-keys is generated from the json key file:

To configure a cluster to access BigQuery tables, you must provide your JSON key file as a Spark configuration. Use a local tool to Base64-encode your JSON key file. For security purposes do not use a web-based or remote tool that could access your keys.

The JSON key file is created right above the following section:

https://docs.databricks.com/external-data/bigquery.html#create-a-google-cloud-storage-gcs-bucket-for...