Things like gravity, and natural selection are just things that happen; we can explain their causes, but(as far as I know) not why massive objects attract less massive ones(or if we can, you can infitely just ask why the causes have the specific effect), et cetera. Does this mean that an author can create a world with a complex set of natural laws, as long as they work it all together, and the laws don't contradict each other?

with a pen and some paper, you can write anything you like.

Yes