If some of you don’t know, you may be questioning why animations from the 40s and 30s were incredibly realistic.

They didn’t make it themselves, they did it with the help of tracing.

this is what wikipedia says:

Rotoscoping is an animation technique that animators use to trace over motion picture footage, frame by frame, to produce realistic action. Originally, animators projected photographed live-action movie images onto a glass panel and traced over the image.