She Who Wants to Catch Fish Only Wants to Catch Bigger Fish in the Apocalypse

The background was not clearly explained and the fight started inexplicably. The highest crystallization of human civilization is a few armor-breaking and individual heroism. Now that we have entered the interstellar society, humans do not even have a space team. The monsters that come are also inexplicable. There is no description, just hit the weak point in the abdomen. The only long-range attack method is a miniature hydrogen bomb, and the technology tree is very sparse. I hope the author can explain the relevant situation clearly and the relevant settings can comply with a certain logic.