I have a Nokia G-2425G-A provided by my ISP(Airtel) which is ONT+Wireless Router. The stock firmware provided with it is not good(in terms of features and options... for eg: No OpenVPN support) and also the settings like PPPoE configuration are locked and SSH access to the filesystem is not supported...
The SSH access to the linux filesystem was blocked but I managed to gain SSH root access by doing a config hack (https://0x41.cf/reversing/2019/10/08/unlocking-nokia-g240wa.html) . So I can get the required info(even files) for building the firmware for it via SSH. The stock firmware use Linux 3.18.21(via SSH)
For some reason it also has opkg version 0.4.0 (via SSH)
The ONT specs and other info is also available at https://fccid.io/2ADZRG2425GA/Users-Manual/Users-Manual-4968412
The model name at fccid is same but somethings like the number of USB and POTS ports, ram and flash storage info differ from the User manual and photos (user manual says there are 2 USB and POTS port but mine has only 1 port each)
And finally an image about the internal functional block from the user manual
Yet I want to build a firmware to directly use on it because the device specs seem good to run the OpenWrt firmware....
Edit: Also I wanted to learn about the building process
Well even if it's proprietary can I just use the stock firmware's SoC driver as there's no public GPON PHY driver anywhere and there is a vendor specific implementation...
No because OpenWrt uses Linux kernel 5.4 (or later). That driver will work only on Linux kernel 3.8.x which is what is currently on that device.
Proprietary drivers on Linux are limited to a specific kernel version because the kernel developers change the driver interfaces often to optimize performance.
Opensource drivers are updated easily to these changes (and are usually updated by kernel developers themselves), proprietary drivers cannot be updated like that so they stop working.