From my reading, I find that the neuroscience community knows some things in a continuum from excruciating detail to a “vague fuzzy feeling.”
From fMRI studies, we can tell things like a given word or activity activates a particular region or regions.
From tract studies, we can tell to a high degree what regions are connected to what regions and the relative density of those connections.
Developmental studies have shown how the connections form in the developing animal.
Lesion studies have done much to catalog and localize functions. Man (via war injuries) has been conducting highly detailed focal injuries experiments to extend this lesion damage knowledge.
From microscopic studies, we have some very good ideas on where the connections are made in the cortical sheet. Perhaps 20% of the neurons in this menagerie have some working theory on what function they provide - many that have strong support via “in vivo” studies. These in vivo studies have received an amazingly powerful tool in light activation and/or light emission genetic manipulation.
Psycological studies have done much to give some good “black box” descriptions of what tasks are being performed by the brain. Other studies have done much to elaborate the order that the brain learns and exhibits these behaviors.
Interspecies comparisons add detail on what functions go with what structure and configuration of the structures.
So - how much is “known” depends to a great degree on how much you are willing to dig and integrate. Whether something is a “preliminary” stage or some more advanced stage may depend on your personal journey. Until someone puts it all together “we” may not know how long that journey is.
It never fails to amaze me that once I get some question in my mind regarding neurobiology I look and - lo and behold - someone has been researching it. There it is - laid out in research papers replete with tables, graphs, measurements, and references.
I think that we are in the same place as chemistry was in 1869 when Russian chemist Dimitri Mendeleev started the development of the periodic table. Once we have the “periodic table of the brain” everything may fall into a framework that makes everything make sense. I predict that we will discover that we really did know the answers - we just did not know how to fit the pieces together.