I believe in some pokemon movies/episodes Ash says the quoted, "What's that" Then before he catches it, it says data on that pokemon.I doubt a ten - eleven year old, no matter how intellegent and dedicated, would know certain things about certain Pokemon. I presume you hear about it from places, such as how you know about those legends, etc.
I presume you, the protagonist, writes the entries, and figures out the location. You are the regions professors helper, after all.
The Pokedex probably scans Pokemon, to tell their weight, height, etc., and records their cries.