GroundedSkybox is the name in ThreeJS of the technique that allows you to fake the floor in a spherical environment like:
https://raw.githack.com/FarazzShaikh/three.js/feat/envmaps-ground-projected/examples/index.html?q=ground#webgl_materials_envmaps_ground-projected
or sky.enableGroundProjection = true; in BabylonJS Babylon.js Playground
Thank you!