Default [Q] New to flashing - Basic questins

This forum is really big and I am new. I apologize if I am asking this in the wrong place. I have been reading a lot about how android working, and serving Google for answers.

I always thought that the only way to hard brick a device was to erase or corrupted the bootloader. Since even if the ROM and recovery had problems, you could use fastboot mode to fix things.

Now I am reading that Roms ussualy include a kernal, and flashing a kernal can lead to a hard brick. Does this apply to the Nexus 7 2012, or just devices that use download mode/ODIN instead of fastboot?

I always thought of a boot loader like an i intersection where you normally keep driving strait (into the main rom), or turn left (into recovery), or stay at the intersection (fastboot). How does a kernal tie into this? Do all Roms include their own kernal, or are different kernals and Roms usually flashed sepretly?

Thank you for helping me to understand this better.