> I'm curious how Microsoft could make certain parts exclusive to its own build of VSCode.

By packaging it as an extension that isn't licensed like the rest of VSCode.

Also on a related note, if you're using an open source build of VSCode (such as VSCodium) then you cannot use marketplace.visualstudio.com/

https://github.com/VSCodium/vscodium#extensions-and-the-mark...