How do I insert an infinite background on my video game??

hello everyone, I'm creating a portrait video game where a spaceship moves vertically upwards thanks to a "Platform" behavior with negative gravity. How can I insert a continuous background that never ends in this video game? I'm not succeeding with the dt.

Thanks a lot to everyone.

Sorry for the ugly scrolling, but it's something like this.

https://drive.google.com/file/d/1X97G_vGQqyx6sZ5W4DX0wU3XmPWs_SV9/view?usp=sharing