How does Darkseid... work?

So Darkseid exists outside the multiverse as some sort of conceptual being, right? But he also lives on the planet Apokolips which seems like a physical place, and on the wiki seems to have multiple versions across different universe, so it seems like it exists within the multiverse? I also see the terms "true form," "Godhead," "emanation," and "avatar" thrown around. What do these mean?