The arch object in Water Park World is part of the Design building options, alongside paths, terrain, walls, roofs, stairs, windows, and doorways.

So in-game, look under the Design tab when you’re placing build pieces. If you saw it in a video or guide, it was likely being used as a decorative or structural piece within that menu.

Quick location

  • Open the building menu.
  • Go to Design.
  • Check the sections for structural pieces like walls, roofs, stairs, and decorative arches.

Note

The search results I found were limited, so I can only confirm that the arch belongs to the Design category, not its exact position in every version of the game UI.