The first two are forks that fundamentally did not change all that much, the latter aims to create a "true" opensource alternative of VSCode (including the ecosystem, where VSCodium falls flat) to serve as a common base for (and by) a few industry giants to build their next-gen of IDEs on top of (an "IDE framework", per their description).
As for the first two, last I checked openvscode-server was just enough to host VSCode for the browser, code-server had a few extras, like hosting at a subpath.
I hate to break it to you but that's a large percentage of open source projects. The high majority of code is by a low minority of people
I think this is because the barrier to meaningfully contribute is too high simply because large code bases are almost always complicated and hard to understand.
There's a few extraordinary exceptions (NetBSD source comes to mind) but they're extreme outliers
https://github.com/gitpod-io/openvscode-server
For what it's worth, we have been using code-server instead in prod (prod being internal) for over a year now.
https://github.com/coder/code-server
https://coder.com/docs/code-server/FAQ#whats-the-difference-...