Don't know about Stellar but the original XRPL consensus which Stellar is based on requires each validator "node" (nodes that participate in consensus finding)

Don't know about Stellar but the original XRPL consensus which Stellar is based on requires each validator "node" (nodes that participate in consensus finding) to define which other nodes it listens to (This list of other nodes doesn't have to be public.) But if you would make all the lists public the most overlapped nodes would be the core. A common misconception is however that nodes in this core somehow have more rights or can somehow affect the outcome of the consensus. Which is not the case. Each node enforces all rules. The overlap is only requires in the sense of that it prevents forking. I.e. if there would be 2 loosely connected "cores" the same size and they get disconnected form each other, both sides could keep going which is objectively worse than if the system halts until the problem is resolved. This "emergency halting" is only guaranteed if there is one core that is overlapped enough so that no 2 separated parts or the network could reach the 80% quorum.