The Linux shell command echo 255 > brightness below turns on a green LED, presumably through I2C bus 1, and peripheral NXP PCA9533...

root@phyboard-mira-imx6-3:leds# ls
input2::capslock@  input2::compose@  input2::kana@  input2::numlock@  input2::scrolllock@  mira-blue@  mira-green@  mira-red@  mmc0::@  phycore-green@  user-led1@  user-led2@  user-led3@
root@phyboard-mira-imx6-3:leds# cd mira-green
root@phyboard-mira-imx6-3:mira-green# ls
device@  power/  subsystem@  brightness  max_brightness  trigger  uevent
root@phyboard-mira-imx6-3:mira-green# echo 255 > brightness 

How do I find the underlying implementation's source code?

Online Linux source code browsers might or might not match what's actually running on this board. Need more hints.