Is the Relay Chain Where the Security Comes From?

In the polka dot protocol, is there a center? Is there a central blockchain with parachains coming off of it? Yeah. The relay chain is like this neutral third party that basically keeps the parachain at its word. So if you look in the context of like nodes, would that be individual nodes making a decision about which role they take or would a single node take on many roles? I'm just curious where those roles really live.