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BigQuery is ACID compliant and easily handles petabytes.

https://cloud.google.com/bigquery/docs/introduction

Yes, but in addition to being ACID compliant (serializable ANSI), you need to support a million transactions per second - it’s not just about data size.

What’s the cloud infra like?

Nobody in their right mind is using BQ for oltp workloads

Generally speaking, you won't find financial institutions using Postgres for core line of business databases.

What will they be using? A similar SQL database that has a big vendor behind it like Oracle, or something else?

Any database easily handles terabytes, idk why that's a condition.

DuckDB and SQLite easily handle terabytes of data. They're just not so much for cloud based apps, or things that need multiusers and access control junk. They're the best choice for just about everything else though.