Here’s the interesting breakdown of what that article likely covers: 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 high above the main map (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."
That is a fascinating deep-dive topic. The "GTA 3 Extended Interiors Universe" (often abbreviated by fans as or just the "Interior Universe") refers to a specific quirk of the game's map design that has fueled modding and lore speculation for years.
