Three js Scene background

const loader = new THREE.CubeTextureLoader()
    const texture = loader.load([
      'https://threejsfundamentals.org/threejs/resources/images/cubemaps/computer-history-museum/pos-x.jpg',
      'https://threejsfundamentals.org/threejs/resources/images/cubemaps/computer-history-museum/neg-x.jpg',
      'https://threejsfundamentals.org/threejs/resources/images/cubemaps/computer-history-museum/pos-y.jpg',
      'https://threejsfundamentals.org/threejs/resources/images/cubemaps/computer-history-museum/neg-y.jpg',
      'https://threejsfundamentals.org/threejs/resources/images/cubemaps/computer-history-museum/pos-z.jpg',
      'https://threejsfundamentals.org/threejs/resources/images/cubemaps/computer-history-museum/neg-z.jpg',
    ]);
    scene.background = texture

const loader = new THREE.TextureLoader()
  const texture = loader.load(
    'resources/images/equirectangularmaps/tears_of_steel_bridge_2k.jpg',
    () => {
      const rt = new THREE.WebGLCubeRenderTarget(texture.image.height)
      rt.fromEquirectangularTexture(renderer, texture)
      scene.background = rt
    })
}

Three.js Backgrounds and Skyboxes

Most of the articles here use a solid color for a background. Adding as static background can be as simple as setting some CSS. Taking an example from the article on making THREE.js responsive we only need to change 2 things.

https://threejsfundamentals.org/threejs/lessons/threejs-backgrounds.html