In the original GTA III, Liberty City is largely a collection of solid, hollow boxes. The Extended Interiors mod changes this by opening up hundreds of buildings, effectively shifting the "story" from the streets into the private lives of the city's inhabitants. Claude’s Life Expanded
In GTA 3 , most interiors (like Joey's garage, Luigi's club, or the hospital) are not physically located inside the buildings you see. Instead, they are rendered (usually around coordinates 0.0, 0.0, or high in the sky) or far below sea level. The game teleports you there via "yellow arrows." Gta 3 Extended Interiors Universe