Themes *.png names or commands

I Wounder how to know the png photos names or commands in emui 3 or any android ver
to name it
for me i extract themes made by others and catch the names so i can change *.png by example battery or network
so my Question how can i know that commands as i would like to make by example calendar widget miui for our emui 3
but i can not fine any theme made by other to catch the names of png for emui 3

What are the prefixes before the phone names?

I always see that some phones have Verizon,AT&T,Sprint etc. names before their model name. What does that mean?

Also, I'd like to buy LG G3 from Amazon but there are so many different products(unlocked,sim-free etc.) that I'm confused. Would you kindly tell me which one should I buy? (I want the 3 GB RAM and 16 GB Internal one)