A monk asked Tonzan, “What is Buddha?”
Tozan replied, “ three pounds of flax.”
-Case 18 from The Gateless Gate
Which is enough flax to make a monks robe. But it's stated as raw material, as before the compounding of fabrication. I think that is worth drawing attention to. I heard this framed as just surrealism, as a refusal of the answer to take the question seriously, that Buddha can be anything, or as saying the question isn't answerable. But that isn't what was happening.
Far from just being gnomic and impenetrable utterances, blank walls to crash the mind against, koans -public cases- often simply assume a deep understanding of Buddhist philosophy that a monk who dedicated their lives to the practice and so hearing and practicing koans would be expected to have. A favourite example of this for me is Dongshan on non-sentient beings expounding the dharma. This isn't simply 'impenetrable to the discriminating mind'. It is deeply engaging with a discontinuity between philosophical strands in Zen.
I strongly recommend Steven Heine's book on the origins of the koan tradition 'Opening A Mountain' (available to download at fhe excellent Zen resources site terebess.hu). This covers the Transmission of the Lamp genre which covered the lineages of teachings through capturing decisive moments and interactions usually of past with future abbots but sometimes confrontations with hermits and Wuist sorcerors. This helps understand how the Zen Golden Age was exactly about asserting this new tradition against other teachings and practices. Then the extraction of decisive interactions as koans. Then capping verses, from the role of these in teaching.
Of course Zen is about 'direct transmission, beyond words and letters'. We should be looking to the moon not the pointing finger - but I think there is a danger of ignoring the form of the finger, and so failing to see where it points!