Can any virus be removed easily on non-rooted phones?

I know that all android devices are hackable whether they are rooted or not. I have general idea of protecting an android phone (or staying safe with android device) from any viruses. What I want to know is curing an android phone. In other words, removing virus on it once it is infected. Are non-rooted phones easier to remove viruses (even backdoors)? For example, by doing factory reset **on** **non-rooted phone**, are all viruses gone? Can malware exist on /system partition or /boot partition **on non-rooted phone**? (In that case, I know I need to re-flash the ROM instead of doing factory reset) I know many known viruses can be detected easily by anti-virus softwares. But nowadays hackers can easily make undetectable malwares. P.S. My phone is not rooted and its bootloader is locked. Can all virus be gone after factory reset on my phone?