Trading.
People trade over the other starters, seeing as you can't get them all within one save file. They may request them from another person, trade with themselves using another copy of the game and another DS (or use Poke Bank for an easier time with it), or get lucky with a Wonder Trade.
Other people may choose to hack them in if they want them that badly and they have the means to, but hacking is less common than just trading for them (or using one starter, the way it was intended).
Hope I helped!