I was a freelance macOS engineer at Sketch from May 2019 to January 2020, working on the plugin platform — the thing that lets third-party developers extend the app, and which is now home to hundreds of community plugins.
The contribution I'd point at is the design for running plugin execution over XPC, in an isolated process. Before that, a plugin was running in the same process as the app, which means a plugin crash and an app crash are the same event. Someone else's bug takes down a designer's work, and the designer is paying you, not them. Moving execution out of the process makes a crashing plugin a recoverable annoyance rather than lost work.
It was a team, and the platform was a team's work — I contributed the design for that piece rather than building the platform.
Day to day it was Swift, Objective-C and JavaScript, with delivery and code review through GitHub.