look into this thread: Blurred reflections
if you use react + three you don’t need to do anything, you just plant a component in there and you have it: Ground reflections and video textures - CodeSandbox
that reflector component was converted into a vanilla class in the thread above, you find everything you need in there.
there is a better way now using the screen-space-reflections library which exports a postprocessing pass. it goes way further and creates realistic reflections everywhere. it used to be noisy but recent updates look quite clean.
example 1: SSR Test - CodeSandbox
example 2: Starwars (forked) - CodeSandbox