Will phone of global market will have better coverage than others? I have LG V20 (H990DS), it's global market's phone. Also there are AT&amp;T, Sprint, Verizon. And I noticed that all models have different frequency bands. Is that mean that my phone will work better around the world and others are fixed to certain regions? If yes, other phones can be unlocked by rooting or something like that to work the same as "Global" phone?
.... my phone will work better around the world and others are fixed to certain regions?


In theory, yes. That's the purpose of a global phone (also called international phone) 


  ....other phones can be unlocked by rooting or something like that to work the same as "Global" phone?


No, since it requires changes at hardware (modem) and corresponding drivers. You can't change hardware by rooting