Editing interface on image file

I'm new on OpenWrt.
I try to create image file for my device U7621. I can get it once it works well but I added some interfaces on LUCI after, I want to edit config files and add these in image.
First like editing etc/config/wireless to create interface for wifi devices

config wifi-iface 'default_radio1'
	option device 'radio1'
	option network 'lan'
	option mode 'ap'
	option encryption 'psk-mixed'
	option ssid '......
	option key '.....'

Also I want to add interface as shown default on LUCI, /like editing cat /etc/config/network

config interface 'UMTS
	option proto '3g'
	option ipv6 'auto'
	option service 'umts'
	option device '/dev/ttyUSB2'

I use the way SDK / make menuconfig for creation of image file.
To get these interfaces as default on LUCI,I could not find the correct files to edit these configuration in openwrt files. Could you show the path of these config files please?

Thank you.


Thank you for quick response.
I got the configuration files from live router by using
mkdir -p files/etc/config/
scp root@xxx:/etc/config/network files/etc/config/network (thanks @jow )
it gives me .txt network file.
Should I edited this file? If it is how? What should be the format of this file?

If you want to make changes to it, yes :wink:

vi ? nano ?

The format is obvious once you edit it.

Actually I want to ask that the format of the file is suitable in this getting form from router .
Since I did configuration on router interface and get the files directly settings okey already.

Besides SDK, is the addition custom files/etc/config/network overwrite the whole default network file?

Thank you