More and more recently, we have begun to see 2D animations integrated into 3D backgrounds, for example, the new Spirit House music video. I think it’s a very lovely aesthetic choice, but I’ve found that its not just a benign decision to make.
Studios may choose to do this because it makes things cheaper. 2D animation for simple characters is easier and cheaper in most cases. However, if you want to have some fun camera moves, it makes sense to have a 3D background made so you can have any angle you like without having to redraw it all.
The more complex characters in this are CG because they’d be a pain to draw. It’s a good mix.