I do not mean a white paper about their layers, like https://www.foundationdb.org/files/record-layer-paper.pdf
I mean a white paper of how they implement the core of foundation db. For example, this is spanner's implementation https://static.googleusercontent.com/media/research.google.com/en//archive/spanner-osdi2012.pdf
I have looked all over but I only see their technical overview here https://apple.github.io/foundationdb/technical-overview.html