Perspective is a wonderful thing. It’s what lets us judge distance. How far away something is, to pick out what’s in the foreground from what’s in the background. It’s why we have two eyes. Cameras, though, only have one. In a still photograph, this isn’t much of a problem. Things just need to be roughly the right size relative to everything else and you can just flat stack 2D images.
When you move a camera, though, for animation or video, flat 2D images stacked on top of each other are an instant giveaway. Walt Disney knew this, so he developed the Multiplane Camera. It allowed him to stack the layers in 3D space so that when the camera moved, we see the shift in parallax and perspective. I saw a small clip of a video on Facebook, so I went hunting for the original on how it works.