>>51612889It's pretty explicit in the original R/G/B that Red and Green are the ones writing the Pokedex entries in that game, since Oak literally tells them that he wants them to fulfill his dream of making a Pokedex by catching whatever they can and recording their findings so he can see it for himself; its also notable that - despite the artwork and anime showing the Pokedex as machines, the original games have him hand Red and Green two blank books when he gives them their Pokedexes (the two books seen on the desk to the top left behind Oak in pic related). The implication is that the two of you are filling out journals that Oak later compiles and publishes as an official book (which, in the original Japanese, is 'ポケモン図鑑,' or 'Illustrated Pokemon Encyclopedia', which seems to imply it's a physical book and not a computer).
There shouldn't be any reason for future protagonists to be writing entries for Pokemon from previous entries, though, so I have no explanation for anything after Gen 1 and 2.
