USB Gecko communicates with a GameCube or Wii via EXI bus. In essence, EXI bus acts like well known SPI.
If command names are confusing to you, read them as if you were issuing commands from PC to console i.e. 0xA Receive Byte
means "read 1 byte from GC".
These commands were documented by studying Shuriken USB
VHDL code. The official USB Gecko
and USB Gecko SE
supported more commands like 16 bit reads/writes or NOR flash access.