The Interior Modpack by Kostič for Bus Simulator 21 is a free utility designed to enrich the customization options available to players. This modpack features 14 unique interior skins that vary in floor and seat colors, allowing users to personalize their virtual bus interiors effectively. The diverse selection includes both ornamented designs and simple colored cloth options, catering to a variety of aesthetic preferences.

Among the offerings are two distinctive 'typical bus' interiors, characterized by unconventional patterns that add a quirky touch to the simulation. This modpack enhances the overall gameplay experience by providing players with more creative freedom in designing their buses, making each journey more engaging and visually appealing.