Luck is not a strategy

Alien species killed 95% of human population. The rest 5% figured out that if you go 2438 meters (8000 feet) above sea level you are fine. The unbreakable monsters can't handle that high an altitude. The mountain forests provide food, some people are creative enough to make bullets and booze. But making medicines is a different issue, so the brave have to look for them them in the monster territories. Maybe gather usuful information of these vile creatures while at it. Essentially stupid and ridiculously flawed little film in A Quiet Place and Bird Box ballbark, but fairly interesting and entertaining. Funny popcorn suspense.