SQL is a protocol. I'm not sure I would consider that "under the hood". What happens under the hood is up to the implementer, and implementations can vary widely (BigQuery is quite unlike Postgres, for example). It is the visible part that makes up a particular public API designed for interacting with databases.
Conceivably even Firestore could expose SQL as one of its APIs. In fact, I once built a tool to do exactly that so I could perform ad-hoc Firestore queries using SQL. Worked beautifully.
Conceivably even Firestore could expose SQL as one of its APIs. In fact, I once built a tool to do exactly that so I could perform ad-hoc Firestore queries using SQL. Worked beautifully.