Spanner can do global consistency and (some?) transactions but I'm unaware of it being able to do the sort of layering Foundation can internally to expose largely different database forms on top of it. I have never used Spanner, though, so I'm open to being corrected.

What do you mean by “forms”? Spanner is also layered. The bottom layer is basically a key-value store. On top of that there’s a full blown SQL layer, which, BTW can work with hierarchical records as well as flat tables. Both support transactions and guarantee global consistency.

They may refer to FoundationDB layers. [1] While Spanner may be built on a key-value store equivalent, Google does not expose it as a service.

[1] https://apple.github.io/foundationdb/layer-concept.html

That's exactly what I was referring to. Thanks.