This video covers 3 methods of making custom skyboxes in Unity3D.
Link mentioned in the video for generating space skyboxes: https://wwwtyro.github.io/space-3d/
The first method is to create or generate 6 images for a cube, and apply those 6 square images to a skybox material.
The second method is to create a procedural skybox material, and modify the colors to suit your game. The default skybox in Unity is procedural.
The last method is to build a scene to be used as a skybox, and use a reflection probe to generate a cubemap. The generated cubemap can then be applied to a skybox material.
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https://github.com/bitgalaxis/Free-Game-Assets
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