In the show it already has most of the data, I don’t think it’s ever elaborated on there either. The pokedex can be used as a trainer id somehow.
In first handful of games it needs to be manually completed. Owning a pokedex is quite rare though so it’s not a crowdsourced task. Not entirely sure why it’s not. It’s made clear that the people with a pokedex at this time are favored by the professors. No connections or fateful encounters, no journey.
During the fifth generation I would say it had changed just based on the atmosphere in the first set of gen 5 games where it’s acknowledged that the professor works with parents to enable their children to go on journeys. In the following two games going on a journey really seems to become a rite of passage, compared to the first four where the player character is very fortunate to have the opportunity.
By generation 8 the pokedex is now a public application that can be put on your phone, meaning it’s probably crowdsourced at that point.
Nah, there was just an option to get the Professor’s rating at the computers based on your progress. It’s doesn’t really do anything, and the feature gets removed later on.
In later games you’d want to talk with the professor(s) directly for item rewards, and the little screen certificates you get for a full completed dex have always come from the fourth wall breaking gamefreak employees in the game.