The files path should be the root path of all your files. For example, if you put /etc/config/firewall in

/home/xxx/openwrt/files/etc/config/firewall, you need to write
“files”: “/home/xxx/openwrt/files”

It will simply copy all the contents to your firmware to override the other files. Note, you have to set the correct access right first. For example if you have a executable script you need to use chmod +x yourshcript.sh in your host system.