See: https://read.engineerscodex.com/p/how-apple-built-icloud-to-...
Discussed on HN at the time: https://news.ycombinator.com/item?id=39028672
[1]: https://github.com/apple/foundationdb https://en.wikipedia.org/wiki/FoundationDB
This is sort of true and sort of false. When you think of a "database", if you're thinking of a Postgres database, you're way off the reality of what "database" means here.
FoundationDB has a concept called "layers", and essentially they have created a layer that looks like a separate database on top of a layer that is separately encrypted groups of keys. They don't have billions of FoundationDB clusters or machines, and at the infra level, i.e. the instances of the FoundationDB server software, it's unaware of individual "databases".
A closer analogy would be like having billions of tables, but even that isn't accurate because in relational databases a table is usually a notably more static concept than data in a table. The closest analogy would be that each of the billions of users has a bunch of rows with a user-id field on them, and there's a proxy that filters everything such that you can view the table as if it only had one user's data in it.
To be clear, FoundationDB is awesome and Apple have done some really cool stuff with it, but it's less crazy/impressive than it sounds.
"FoundationDB decouples its data storage technology from its data model. FoundationDB’s core ordered key-value storage technology can be efficiently adapted and remapped to a broad array of rich data models. Using indexing as an example, FoundationDB’s core provides no indexing and never will. Instead, a layer provides indexing by storing two kinds of key-values, one for the data and one for the index."
https://apple.github.io/foundationdb/layer-concept.html
Then existing standard layers like the Record layer, providing "(very) roughly equivalent to a simple relational database" providing structured types, index, complex types, queries, etc.
https://github.com/FoundationDB/fdb-record-layer
Or one for documents, which speaks the MongoDB wire protocol