Actually, the prologue to the game is about Mari's life before her emotional breakdown, her discovery of the Gensou MMO by the introduction of her friend Karen, and her first days of using the Gensou client.

In the AU, after the Sages of Gensokyo decided to build a MMO in order to help bring all residents and all races of Gensokyo together in a similar manner to MMOs in the Outside World. The idea is developed by Yukari Yakumo, who turned out to be travelling to the Outside World many times and know the trends out there. But while the Sages are stuck with the idea who needs to code the game (since Gensokyo does not have any notable residents who know computer coding), Chiyuri Kitashirakawa (yes, THAT Chiyuri) came out and decided to present her longtime MMO project, made since her first days of returning to Gensokyo after leaving the Outside World: the Gensou Rhythm Battlefields. The Sages (mainly Yukari) approved of Chiyuri's project, and allow it to be distributed all over Gensokyo. But since Gensokyo have no hardware for running online games, Yukari asked the kappa engineer Nitori Kawashiro in order to create consoles built specificially for the game. Despite she never created a gaming console before, but with Chiyuri's documentation on modern computer engineering and software engineering, finally Nitori is able to create a line of consoles built specificially for Gensou, and these consoles are distributed at Kourindou.

The above is somewhat like a summary of how Gensou is created, along with the consoles that are used to play it. For me, story ideas are easy to think up, but writing it down can be tedious for me, since I still have half-baked writing and grammar knowledge.