After completing the campaign, the boss eventually escaped and all that was left was PVP
So we can think of the plot as humans constantly killing each other after acquiring extraterrestrial mecha technology.
Or even worse, humans have long been extinct, as you can see many buildings and ruins in the campaign scene, but no humans appear. The ones who kill each other are artificial AI mecha.
Anyway, these are just my guesses. I hope there will be more rich storylines in the future.