Hello @ZoeLeee , how are you doing.
Yes, environment textures and mirror work together. You just have to assign the ground generated from createDefaultEnvironment to the renderList of the mirror texture.
Here is you playground example updated:
Mirror Example | Babylon.js Playground (babylonjs.com)
Is that what you are trying to accomplish?
Let me know you need anything else.
This is great, it’s not what I wanted, it gave me new ideas, I clone a mirror texture for the ground and the environment texture for the original ground. But I think there should be a better way.
What exactly do you want, then? It’s very hard for us to understand your needs without an explanation of what they are. 
But if you want to have environment + reflections the ReflectionProbes are a great way to accomplish that too: Reflection Probes | Babylon.js Documentation (babylonjs.com)