The short version is all of them use different processes to get to the final result. And what's complicating things even more is that the order in which things are processed. Darktable uses the same process colourists from the video and cinema world use. Thanks to that, Darktable is significantly more consistent and predictable than its competitors. Because essentially how other raw editors process a raw is, they target the "casual" audience and they want good looking results fast without any sort of user input. So they essentially just slap a gamma curve and a lut on your file and then you can only make your adjustments on top of that. This is essentially just editing a high quality jpeg at this point. Worst of all, most RAW editors do not even give you control over this base curve or base RAW lu. This is bad because it's curves upon curves upon curves upon curves, and these errors multiply over each other, so good luck controlling the output.

Now on the other hand, Darktable not only exposes everything from raw black and white points, demosaicking and any sort of other adjustments that you'd like, but also, thanks to the scene-referred process, it defers the Gamma Curve to the last step, which essentially just means that everything happens in scene-linear space and your adjustments are linear and predictable. No curves multiplying errors with curves. Plus, you can fully control the gamma curve and this guarantees that the entire dynamic range of your camera gets output to the display, which is not the case for other raw editors which use the other process with curves, the so called "display referred" process.

All of this may seem very complicated, but using Darktable is really easy, because it by default gives you a well thought out process for handling your raw files, which gives you consistent and predictable results. Darktable is free software and available on GNU,/Linux, macOS and Windows.

TLDR: Darktable hype.

Edit: I accidentally deleted half a sentence and it awkwardly melded two sentences together. I corrected it and I apologise for my mistake!

Yes. The decoder decodes. There are several open source ones, libraw, rawspeed, rawler. They all do the same thing.

At least for rawpseed, things aren't always straight forward, as all camera support is reverse engineered. Don't have data from the manufacturers as its all locked up behind NDAs and such