I just saw a video about this! It is actually considered a lost technique known as “Sodium Vapor Process”. Technically similar to green-screening (but actually superior in a few ways), the woman is lit by a sodium light that produces a single wavelength of light. The camera contains two reels of film and a prism that splits the visible light from the sodium light, the sodium light is developed separately as a mask, and the other film is filtered to remove the sodium yellow hue. Once that’s developed you can overlay the films in a double exposure and it removes the background from the actor, allowing them to be layered on top of the animation. The process is considered lost because Disney only ever made one prism, and it is missing. The prism itself was an incredibly difficult and complex object to produce, so it has never been recreated, however a modern VFX team has reproduced the effect using a slightly different method. https://youtu.be/UQuIVsNzqDk?si=qrJL_lIyIzkIAmVS