I think this bit from "Borg, Omega, and Kubernetes"[1] (which is an excellent read) sheds light on this:

The Borgmaster is a monolithic component that knows the semantics of every API operation. It contains the cluster management logic such as the state machines for jobs, tasks, and machines; and it runs the Paxos-based replicated storage system used to record the master’s state.

So it sounds as though Borg includes its own storage system. As I understand, Google has a set of (very complex) libraries written in C++ that implement Paxos/Multi-Paxos[2], which they have not open sourced.

[1] https://research.google.com/pubs/pub44843.html [2] https://research.google.com/archive/paxos_made_live.html