The server boundary
Roblox experiences are partitioned into servers with hard population caps, and those boundaries are visible in the fiction. Two players in the same city may be in entirely different worlds, and nothing either does affects the other.
Quantum-7 investigates presenting multiple servers as one continuous simulation, so a persistent world can hold far more concurrent participants than any single server allows.
What actually needs sharing
Full state synchronisation across servers is not viable and is not necessary. Most state is local and irrelevant elsewhere. The research is in classifying state by how widely and how quickly it needs to propagate.
Economy and world-level events need global consistency and can tolerate latency. Player positions need low latency and only matter locally. Those different requirements suggest different mechanisms rather than one synchronisation layer.
Consistency and handoff
Any distributed system must choose what to sacrifice when parts of it cannot communicate. For a game the answer is usually availability over strict consistency: continuing to play with slightly stale information beats stopping.
Moving a player between servers without a visible break is the other hard problem, requiring their state to exist in both places briefly and the handoff to resolve cleanly if it fails midway.