By now, you already know that there's a particularly hyperactive tendency among Pixar film-makers to include Easter Eggs and call-backs (and sometimes forwards) to their other movies. It's as big a part of the Pixar experience as it is watching a Marvel or DC movie as directors race to hide as many homages and references knowing that the audience demand more and more reasons to rewatch the films.
There's even a sort of Pixar Easter Egg vocabulary - including the Pizza Planet truck, the Luxo Jr ball and A113 - which is seen in every Pixar film (in increasingly devilish ways). That's how aware the studio's creatives are of the culture of reference hunting.
But they're the easy ones, and so too are the obvious cameos and nods that happen in the foreground action. The real art is in hiding little details in background detail that will rarely be noticed the first time anyone watches the movie. The real genius comes in making them so rewarding to find that fans then rush to share their discoveries online with fellow obsessives - basically doing Disney's marketing campaign work for them, even well after the films have come out.
You might think you know Pixar by now, but they keep throwing up surprises hidden in the background of even their most famous movies...