he can also reuse backgrounds and not spend time rendering them again or recreating them?
If he had to recreate them for each shot ... that would be really suicidal
However ... the layers with the characters often contain a little bit of background, after all they do reflect each other a bit. (just as clothing layers also contain skin)
This implies that he renders the complete image, then cuts out the parts that differ. And he does not get the lighting right every time. At least in an early version you could see that in the scenes with Alice on the couch, the couch keeps changing color/brightness.
But for the random encounters like changing scenes he usually does not include background in the character layer, that's right. They may be indeed rendered before a black background, the borders are sometimes a bit too dark.
People tend to overestimate the time it takes to render an image. The preparation is the complicated part, setting the positions, lighting and so on. And in this case there's also post processing (cutting out layers with characters and clothes, organizing them in the file structure, making them usable in the logic) The rendering itself you can do overnight, once you set up a pipeline with all the images in the queue.
