From the Super Mario Wiki
Shroom City is a large city located on an unknown island, which also consists of a city built on the islands below.
After the player finishes making their passport, they meet Toad, who tells them about all of the fun activities in Party World. Suddenly, Bowser and Koopa Kid drop in and squishes Toad, scattering Gaddgets and minigames to Shroom City. After they leave, Toad tells the player to collect the missing Gaddgets and minigames from Shroom City's locals.
In Mario Party Advance, there are fifty quests for the player to solve in order to get a Gaddget from a citizen or a minigame from Bowser. Bowser can only be confronted after certain numbers of quests are completed, after which he will open one of the Once a certain number of quests are completed, Bowser allows passage through one of the pipes, letting the player confront him. Once all the citizens' quests are complete, the Pipe House can be visited to confront Bowser for the final time.