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.
ConstraintLayout is an Android layout known as a view group, which acts as a container for laying out UI elements. It uses constraints to build large and complex layouts, while maintaining a flat hierarchy of views.
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.