I don’t really use either (outside of work scenarios where its going to be regular VSCode on windows anyway), just going off memory here, so I’d need to check too.

Doing a bit of looking, per vscodium folks:

They are very similar. Code-OSS is what you get when you build vscode from source. VSCodium is essentially just a build script that automatically builds from source when MS cuts a new release and then uploads the binaries here to GitHub. In that sense it is mainly to save time.

Additionally, VSCodium turns off telemetry in the build process, and rewrites some of the deeply nested telemetry URLs to go nowhere in case something in the codebase tries to send info back to MS. So that is a small difference that a standard build of Code-OSS would not have unless it was done manually.

I’m not sure how the packaging was done to get Code-OSS into Arch, so it’s possible there are other differences with the Arch version specifically.