The columns with common dtypes are grouped together in something called "blocks" and inside those blocks are 2D numpy arrays. It is probably not in the documentation because it is seen as implementation detail but you can see the block manager's structure in this article (https://dkharazi.github.io/blog/blockmanager/) or in this talk (https://thomasjpfan.github.io/scipy-2020-lightning-talk-pand...).