At least with the rockchip-based designs you can get the TRMs for the SOC.

https://github.com/Hao-boyan/rk3588-TRM-and-Datasheet https://opensource.rock-chips.com/wiki_Main_Page

For the raspberry Pi 4b, the publicly available documentation for the BCM2711 SoC is laughable, barely 160-odd pages long and missing key details (such as which timers are accessible from the ARM core, and which are accessible from the GPU).

P.S. if anyone knows where to find the full TRM for the broadcom BCM2711, I'd be really grateful if you got in touch or sent me the pdf.