At Apple where he worked for nearly 12 years, he built developer tools like Swift and the tools used to program in the language, like Xcode and Swift Playgrounds. In other words, he focused more on tools for iOS engineers, rather than iOS apps themselves.
https://www.businessinsider.com/sifive-vp-chris-lattner-apple-tesla-google-2020-2
https://www.businessinsider.com/sifive-vp-chris-lattner-apple-tesla-google-2020-2