Thanks to a tip from user clever in our Discord channel (#rpi4-uefi-dev), I just found out one of the Pi Foundation engineers and forum moderators, jamesh, posted something cool:
Enjoy this extensive read during the current crisis! It’s a major update to the original 2835 datasheet, with a lot more information. If you find any problems, please comment on this thread.
https://www.raspberrypi.org/forums/viewtopic.php?f=63&t=270102
https://www.raspberrypi.org/documentation/hardware/raspberrypi/bcm2711/rpi_DATA_2711_1p0.pdf
This doc is for the SoC in the Pi 4B. I personally haven’t read through it yet, but glancing at the table of contents I see there are many goodies here. Hooray for not having to look at Pi 4B DTS (device tree) sources as documentation! 😇