It sounds like you have not looked at the docs:
To answer your next question, this means that Hubitat has either tested the device to work with built-in drivers (or written one specifically to work with the device) or at least heard a consensus from the community that it does.
Before that list existed, early adopters noted their findings here instead: Custom Device Drivers [Wiki]. This list has continued to be maintained (by the community) as a list including community-created drivers, but Hubitat, by definition, does not provide support for community code. However, it may still be useful if you're looking for a device you can't find any "official" word on. So could a search of this forum or a post of your own asking a specific question about a specific device.
I'm not sure how your conclusions follow from your proposal in the rest of your post (e.g., I understand you must want the drivers to be open-source, though they happen to not be; but this would still not answer every question you could have about any particular device unless you also know how its firmware works, possibly inner workings of the platform itself, and other details). If you can provide an example of what you're actually looking for, or perhaps another commercial hub that does what you're asking, it may help clarify.
