Pixar subtly confirmed its shared universe theory 12 years ago in one of its worst movies. Pixar’s shared universe theory is based on this idea that every single one of Pixar’s movies exist in the same universe. Primarily, it's based on Easter eggs placed throughout each movie, such as Brave's wooden carvings of Sully from Monsters Inc. Also, the famous Pizza Planet delivery truck appears in nearly all the Pixar films.
https://screenrant.com/pixar-shared-universe-theory-cars-2-up-shared-background/