What Animation techniques did Walt Disney Studios came up with?
I know Disney first make cartoon movie 60+ minutes (use public domain story) and created list of 12 principles of animation influence animation industry. But Disney Corp probably not first use these principles. 1914 Gertie Dinosaur animation use most principles already. Felix Cat cartoons from 1920s also use repeat background motion like Hanna Barbara, and use correct joint motion for characters instead hose style in later 1920s, animation cycles, and early 1930s (Felix Cat: Stone Age 1922). Fleischer Studio first use sound 1926, two years before Disney. Fleischer Studio also use real people with animation in the Out Of The Inkwell series (1918-29) and this series use limited animation method only redraw changing parts of characters (Hanna Barbara use this method from late 1950s).

Book Illusion Of Life tell about Disney animation invention and experiments with different animation effects (authors two Disney animators, maybe have bias). But I think Disney improve art quality and fluid motion in animation more than other company before.

Disney is probably first and best for cheat public domain (after take much from public domain) prevent their character become domain by buy politician in US Congress for extend their copyright (monopoly) two times.

Some modern methods at Disney Research (research paper):
https://www.disneyresearch.com/publication/