GitHub clients have long been a favorite open-source project for iOS developers. CodeHub was the best known and is now marked as unmaintained, as are GitHawk and GitBucket. iGit uses GitUpKit to clone a repo and make commits on device, GitPoint is a React Native client, and SwiftHub is a reference app for RxSwift with MVVM.
Smaller tools cover one job: Pocket Trailer tracks pull requests and issues, GistHub manages gists, GitTrends tracks repo clones and views, and Contribution Graphs for GitHub shows your activity as a Home Screen widget. ZLGithubClient uses both the REST and GraphQL APIs. These apps are useful for learning OAuth, pagination and caching against a well documented API. Expect older ones to need updates for GitHub's current authentication.