Have you ever thought that how these applications in our phone get updates? The answer is simple it is a Version Control System.
So, whenever there is an update in an application, some changes are made in the program and the old version is kept in the former control system.
Git and GitHub are two different platforms and do have a relation with each other.
Git is a version control system that lets you manage and keep track of your source code history. GitHub is a cloud-based hosting service that lets you manage Git repositories. If you have open-source projects that use Git, then GitHub is designed to help you better manage them.
I have seen many people facing problems regarding putting Git in the system using command prompt. Even GitHub realized that made things easy for their users.
Github Desktop is an application by Github . It has made things like pull request, merging request and edits in code very easy. You can directly clone the repositories from the projects on Github using it and edit it.
Atom is a free and open-source text and source code editor for macOS, Linux, and Microsoft Windows with support for plug-ins written in Node.js, and embedded Git Control, developed by GitHub. Atom is a desktop application built using web technologies.