I'm almost certain that girlfriend is дівчина when possesed by someone.
My understanding is that Подруга is just the feminine form of "friend", but can also be used to refer to a girlfriend. I'm scared that I may someday say "this is my [female] friend (подруга)", but it will get interpreted as girlfriend :D But yeah, дівчина is always "girlfriend" IIRC.
"Моя подруга/ приятелька" = "My female friend"
(platonic); and, "Моя дівчина" = "My girlfriend" (romantic). I hope this helps. :D
Хіба it is можна використовувати з людьми? Я думав для живих істот тільки this is