@Stefan Sorry forgot to address this: there is a steadily improving developer documentation available under the Wiki entry in this forum. The most relevant one currently is probably Internal implementation of Pools, Shares, Snapshots and Clones. But I’m not sure if this covers the initial boot process which may be of most interest. We already spoke about the official user documantation in another thread and lastly there is of course the source code itself on GitHub.
Hope that helps and if not then do please propose what you find to be missing and an appropriate issue can be opened ready for whoever want’s to chip in.