Atom was an editor made by GitHub that competed with Sublime.
After creating Atom, GitHub pulled the editor guts out of it and initially called it "Atom Shell". https://github.com/mapbox/atom-shell
This then had a name change of Atom Shell to Electron to decouple Electron from Atom (the editor). https://www.electronjs.org/blog/electron
Microsoft built several key tools on top of Electron (VSCode being the relevant one here) and became very interested in maintaining control of it... and so bought GitHub when it was up for sale.
Eventually, Atom was sunsetted. https://github.blog/news-insights/product-news/sunsetting-at...
> Atom has not had significant feature development for the past several years, though we’ve conducted maintenance and security updates during this period to ensure we’re being good stewards of the project and product. As new cloud-based tools have emerged and evolved over the years, Atom community involvement has declined significantly. As a result, we’ve decided to sunset Atom so we can focus on enhancing the developer experience in the cloud with GitHub Codespaces.
> This is a tough goodbye. It’s worth reflecting that Atom has served as the foundation for the Electron framework, which paved the way for the creation of thousands of apps, including Microsoft Visual Studio Code, Slack, and our very own GitHub Desktop. However, reliability, security, and performance are core to GitHub, and in order to best serve the developer community, we are archiving Atom to prioritize technologies that enable the future of software development.