This is a paid asset, but now you can download Easy Open World Free.
Detail this asset from Unity Store: Original Link
Easy Open World v1.2 (Latest version)
This tool allows you to get quickly started with a seamless open world game.
The world is split into smaller scenes that are automatically loaded around the player. The size of the world and tile size can easily be adjusted to your needs. One base scene is always open that can be used to hold larger elements like terrain. Terrain can also be split into different tiles (see preview video).
Custom editor UI is included to easily navigate the map and load/unload tiles.
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Update 1.1
– Added a Fill button to automatically add empty scenes to fill the map
– Added a UI offset variable so you can move the UI not to overlap with other assets