First, you are making a common mistake thinking it is 36 bit. I made the same mistake for a while. In reality, RAW data is monochrome and thus only 12 bit in your case since each pixel doesn't have any color information without looking at neighboring pixels.

Beyond that, it depends on the software being used. Color, as mentioned, is derived from the color of the filter on that pixel and the value of neighboring pixels of other colors, but the pattern used can vary.

Similarly, the reduction in bit depth varies even more. It could be a linear map that brings the darkest to darkest and brightest to brightest. It could just grab the middle. It could try to make processing judgements about what a dark black point and what a bright white point should be and adjust according to that. It really depends on how the software decides to do it and then how you adjust the mapping during development.

And that's really the point of RAW. It's designed to allow you to make selections about how to do that mapping as the photographer. If you just want an automatic process to form an 8 bit file for you, simply shoot JPEG. Using RAW is a waste of space. The point of RAW is that it lets you control the process of converting it to an 8 bit space by hand, and thus ensures you get the information you want out of it.

As for why it seems drab initially, it is probably just a stylistic thing for how the logic works. With Lightroom, it tries to make choices to make it look much more like a JPEG would by default, but adjustments are still needed in either case. That initial adjustment is going to vary from software to software and camera to camera and even photo to photo potentially.