its a thing ninty does

basically, to warrant the sale of two seemingly identical games,rather than just one, nintendo has made some pokemon catchable on one version, and others on the other. that way, for you to catchem all, you would have to trade(which meant cables in the past --> more money for nintendo)
i dont think its possible for you to have all 107 new pokemon from just one game by going online and it is impossible because the games have different legendary pokemon. anything you do, you cant get it

the only difference besides that might just be some small changes which dont have much affect on the game