Looking through the page and the main readme I have lots of questions...

I'm vaguely curious what is or is not missing for aarch/riscv, also what else is/is not implemented? e.x. is there audio support? can I plug a USB drive in?

Also, Is there a section with documentation on the architecture?

Author here. aarch64 is something I haven't looked into yet. Last time I tried, I was able to boot to userspace on a VisionFive2 (riscv64gc), but it's been a while and the port has likely bit-rotted. It's missing a lot of platform drivers since there is no ACPI firmware on those devices. There is no audio yet, but I do plan on porting Pulse with OSS drivers in the future. Regarding USB, I have a local change set with HID on xHCI, but no Mass Storage support.

There doesn't seem to be audio support (I searched for audio and sound). There seems to be the beginning of USB support (under drivers/usb/xhci, for example).