Some Questions about kernels backporting and gcc errors

[LunL Lee]

Hi, I'm absolutely new in touching Ubuntu and modifying sources. I'm using a phone which doesn't have any roms supported and only 5.1.1 firmware is out in stock.
Anyway, I have my kernel source based on linux 3.10.49 right now and having hard time googlimg for backporting. One dev I know personally advised me to backport the modules from newer kernels and insert into exisiting old kernel. My exact situation is I have 3.10.49 kernel source right now (which was used for 5.1.1) and want to make 6.0.1 android version compatible kernel. So here's my questions.

1.What do I have to backport drivers exactly? Can I only just backport the drivers that are used in my phone? Or can I just download the backported stable version and put all?
2. What is the way? so i mean how do i exactly do the thing "backporting"? I know what it means but I cannot figure it out how to do it. I have to make backported modules.
3. According to one I got advised, he said that I need to manually port other modules too (not only drivers) but I cannot notice what kinds of modules are in my kernels because I always get implicit declaration errors. If I fix this gcc related error, is it able to view module lists?

4. I think it is gcc version errors so I tried to change the versions into 4.9 and gets new error in makefile: multiple targets. How can I fix this?

My phone is lenovo PB1-770M. Looking for your help! Thnx!