Gordi
To my understanding the main difference was license.
When we (Microsoft) build Visual Studio Code, we do exactly this. We clone the vscode repository, we lay down a customized product.json that has Microsoft specific functionality (telemetry, gallery, logo, etc.), and then produce a build that we release under our license.
When you clone and build from the vscode repo, none of these endpoints are configured in the default product.json. Therefore, you generate a "clean" build, without the Microsoft customizations, which is by default licensed under the MIT license.
Source: https://github.com/Microsoft/vscode/issues/60#issuecomment-161792005
So Microsoft's version makes some minor changes and is released under a different license.
Visual Studio Code flatpak should be unmodified Microsoft version. It states:
This is the proprietary Microsoft build of Visual Studio Code, packaged into a Flatpak. This repackaging is not supported by Microsoft.
Source: https://flathub.org/apps/details/com.visualstudio.code
But Solus had absolutely nothing to do with the flatpak's creation.
VS Codium to my understanding just disables all telemetry that the user could have disabled themselves in settings. The only real benefit over MS Visual Studio Code is that its under the MIT license (Same as Solus vscode) instead of a proprietary license.
To complicate matters further recently MS introduced some more nonsense that broke functionality that they refused to fix saying it was a design decision. So their crash reporter was removed entirely from Solus's vscode https://dev.getsol.us/R3148:7a9127428d89ac3f47e8766898d8f72a3f3aa885
Probably didn't make that any clearer huh?