The way Relatr implements this is as follows: on the first server run, it takes the source public key defined in the config and the configured hops. It then starts scraping relays to obtain follow lists. Once all contact lists for the defined hops are scraped, it builds the social graph (currently using Martti Malmi's library). After that, it begins to compute the defined validations. Once this process is finished, the system is ready.

At this point, if someone requests the trust score for a public key not in the graph, Relatr fetches the contact list, performs validations, and returns the computed trust score. This new public key is then added to the social graph, and the validations are cached, so this is just done once. Relatr does not attempt to build a global graph, however, you can define as many hops as you want to get a more complete picture. Even in this case, there will be instances where new public keys appear, and you'll need to compute the trust for them.

I hope this clarifies the process