I found this part of the documentation here, that classifies peer discovery mechanism as either Ambient or Active.
Under Active Discovery Mechanism, we have listed: Content/Peer routing, DHT random walk and Rendezvous servers are listed.
I sort of get what the DHT random walk is about and how to configure it using the js-libp2p-kad-dht module. What I am not sure about though are Content/Peer routing and Rendezvous servers? What are the concrete examples of these discovery mechanisms? and how to use and configure them?