Although the first meeting between Rocket and Tony Stark had the latter mistaking the talking raccoon for a "Build-a-Bear," a later scene in "Avengers: Endgame" shows Rocket, Tony, and Professor Hulk (Mark Ruffalo) all working together to construct a new Infinity Gauntlet using Infinity Stones gathered through a "Time Heist."
If Rocket and Tony were able to collaborate on a project of this scale (one which literally decided the fate of the entire universe) it stands to reason that they might have collaborated on a smaller-scale project as well –- particularly because both were alive during the five year gap known as "The Blip." Even if Rocket built this by himself after Tony's death, the technology present in Nebula's new arm certainly seems like it was pulled straight from Tony's own designs. Of course, unless we see this occur in a flashback or it's referenced in a later Marvel project, it's hard to know if this was with Tony's permission or not.
It's also worth noting that several fans online initially believed this arm was repurposed from the mechanical arm of Bucky Barnes (Sebastian Stan), which was gifted to Rocket during "The Guardians of the Galaxy Holiday Special", though James Gunn once again shut this theory down on Twitter. Although we may not know exactly where this arm came from, it would certainly be nice to know that Tony Stark's legacy might live on through Rocket's gift to Nebula.
