This is a general map of where most important files are located:

This folder contains files and folders that will be integrated in the firmware’s /etc folder. These are its subfolders and files:

• board.d/ = scripts for defining device-specific default hardware, like leds and network interfaces.
• hotplug.d/ = scripts for defining device-specific actions to be done automatically on hotplugging of devices
• init.d/ = scripts for defining device-specific actions to be done automatically on boot
• uci-defaults/ = files for defining device-specific uci configuration defaults
• diag.sh = defines what is the led to use for error codes for each board

This folder contains files and folders that will be integrated in the firmware’s /lib folder.These are its subfolders and files:

• ramips.sh = human-readable full board name associated to script-safe board name
• preinit/ = common <arch_name> preinit startup scripts
• upgrade/ = common <arch_name> upgrade scripts

This folder contains files and folders that will be integrated in the firmware’s /sbin folder, usually common <arch_name> sbin scripts and tools.

Device tree source files, or dts for short.

Configuration needed to build device-specific flashable images.

Board-specific configuration.

Arch-specific kernel module config file for menuconfig

References:
https://lede-project.org/docs/guide-developer/adding_new_device