Not talking about the filesytem here, but the hierarchy in a Linux environment.

• /opt and /usr/local:
  • https://unix.stackexchange.com/questions/11544/what-is-the-difference-between-opt-and-usr-local
  • https://askubuntu.com/questions/34880/use-of-opt-and-usr-local-directories-in-the-context-of-a-pc
  • https://refspecs.linuxfoundation.org/FHS_3.0/fhs/index.html

mount -t overlay overlay -o lowerdir=/lower,upperdir=/upper,workdir=/work /merged