My guess is that it's pretty much just BigQuery. No one else seems to be able to compete, and that's a big deal. The companies moving their analytics stacks to BQ and thus GCP probably make up the majority (in terms of revenue) of customers for GCP
Given how cheap bigquery is, there would have to be a lot more bq-only customers than customers that use other services. And given how seamlessly the different products work with each other, any beachhead product like bq will quickly garner more product usage.
I doubt it. Not only does Apple (maybe?) run one of the largest Cassandra clusters in the world, but surely they wouldn't leverage cloud provider features over open source alternatives for fear of vendor lock-in.
Cassandra and BigQuery are not at all comparable. BigQuery's open source competitors are Impala, Presto, and Drill.