Root in fastboot mode explain

When rooting does it matters with the security ? let say like LG G6 H87211G can I extract firmware from H87210F and root without any issues or it works with OS version mostly i would like to read the firmware from the phone and extract boot.img from it but any tool I use it save the file in .tot format and can't extract or open .tot I try using eft but does not support android 7.0 if there is any tool to open .tot or convert .tot to kdz will be okay.
LG can easily be killed by downgrading