GitHub Codespaces is essentially a rebranded and more github integrated version of Visual Studio Codespaces.
You can get a pretty good sense how this will all work by reading this post of mine, and the VS Codespaces documentation.
Broadly speaking there are two ways to work with this: you can run a version of VS Code entirely in the browser that is connected to a cloud hosted computing environment, or you can connect your local VS Code to that same cloud hosted computing environment.
I don’t see any indication that Atom devs have been moved to codespaces, as far as I can tell codespaces is a product that really originated from the Visual Studio team.