What copyright license is content posted on this instance (or Lemmy in general) assumed to be?

Since Reddit is now explicitly planning to sell user generated content for AI training. It got me thinking about Lemmy.

What license are posts and comments assumed to be under on this instance? Is there an overarching lemmy policy (there doesn't seem to be)?

Is it down to the user to specify, if so how?

Are there any downsides with adopting a Creative Commons or other copyleft license?