Pixar has been charming audiences since 1995, and in that time, Pixar fans have come up with all sorts of ways to explain how all the movies are connected, including one overarching theory that the magic seen in "Brave" led to superheroes seen in "The Incredibles" with the villain Syndrome's A.I. technology resulting in sentient toys as depicted in the "Toy Story" films, and that all eventually leads to a war between humans and animals. (No, really, there's a whole website dedicated to it.)
While not confirmation of that particular theory, the animators at Pixar have worked in small, hidden references, or Easter eggs, that harken back to a previous film, in each movie. The video above tracks the references, starting with the most recent film. So, in "Finding Dory," there's a quick cameo from Riley, the human protagonist of "Inside Out," a film that features a statue of Forrest Woodbush, the Styracosaurus that appears in "The Good Dinosaur." And the Easter eggs keeps piling up from there, a sort of nesting doll of visual references.
It seems pretty unlikely that these little callbacks are intended to be anything other than in-the-know jokes between filmmakers and their audience, a way to reward those who really pay attention to the movies. But it doesn't necessarily mean the films aren't all in the same universe, either. In which case, well, it's cool that all the humans left Earth on the Axiom so cars could have romances.