> Those manuals indicate just a fraction of the complexity that lies inside of the device -- they describe features that I can use, not even how those features

> Those manuals indicate just a fraction of the complexity that lies inside of the device -- they describe features that I can use, not even how those features are implemented or made. And even then: 1) There's parts of the manual which are brief summaries, written with the assumption you're already familiar with the larger standards they interface with. The SDIO and USBOTG sections of the reference manual come to mind in particular, not to mention all the external manuals it takes to describe the ARM Cortex-M architecture. 2) That's a fairly simple microcontroller. (Given the page count, it sounds like you're working with STM32F401.) The STM32H7 series reference manual is 3500+ pages.