The first sentence is automatically generated. As you can see it all follows the same format, "$name is a $type type Pokémon introduced in Generation $gen" etc.
For the newer Pokemon (Gen 6-7), I took some bits of text from the official site/press releases. TPCi started adding longer descriptions, which presumably the Pokedex entries are taken from as they are similar.
However, a while back I also started writing custom descriptions. I asked people on here for help with writing them but I have yet to get around to adding them. I still have the document somewhere and will do it eventually! I wrote the Scizor one as an example, and I also wrote some custom stuff for older Pokemon that have changed type like Mr Mime.