I think they are using image segmentation with Deep Learning. The technique should be similar to this. Of course, there will be some traditional CV techniques as well.
I think many mobile device/camera apps have been doing this. The portrait feature where the background bokeh is artificially made is using a similar technique.
https://github.com/anish9/Fashion-AI-segmentation
Impressive idea and execution! How could I never thought about this? :)