From what I read, it's a Plan9 fork refreshed for modern build chains plus golang support. From floren's comments, it sounds like the architecture is still Plan9.

It's a Plan 9 fork, the Plan 9 papers and man pages would describe the system architecture pretty well.

Most of the changes I know of that they've done (UFS support, ELF executables, standard C without the Plan 9 extensions, a different build system, etc..) are more in the details than the architecture.