Bitrise is a cloud-hosted CI/CD as-a-service platform built for mobile application projects. It is a mature product, and is widely used for mobile engineering projects. It does have some small disadvantages over Circle CI and other build platforms (see below), but the built-in mobile features make it worth consideration.
Cross platform development is a technique for developing Android and iOS apps using a framework which allows a single codebase to be built into a fully native app for each platform. Popular frameworks for cross platform development are Flutter, React Native, Xamarin and Titanium. (This article does not cover frameworks such as Cordova, which use a single codebase but compile to wrapper apps on each platform). Each framework uses its own development language, e.g. JavaScript for React Native and C# for Xamarin.
SnapKit is an Auto Layout DSL (Domain Specific Language) for iOS and OSX. It allows the use of written code to lay out views in Swift, as opposed to Apple's GUI-based Interface Builder tool. In iOS development, views are your UI elements and constraints are the relationships between views.