If nihilism stakes it's claim that nothing has any meaning on the fact that existence is temporary and nonexistence is inevitable how do they justify taking any actions whatsoever? How do they justify self preservation? How do they reconcile the existence of their own motivations or goals, even if they're primarily biological?

You don't get to say "lol nothing matters" and then also take action towards specific desired outcomes. Hell the presence of a desired outcome at all is anti-nihilistic.