With some frequency, I see posts that I would say could have been in either queue. Is it decided
- somewhat randomly?
- with one of the queues in priority?
- by some maybe indirect trigger?
- in a complicated way?
By 3, I'm thinking of, say, a person who is not a New Contributor, deletes their posts, therefore becoming a New Contributor again (as explained in this answer); yet the system knows not to place them in the First Posts queue. I would hope it's like this, because I tend to be more lenient in my voting/comments for posts in the First Posts queue, and I don't really want to have my words fall on ears proven slightly deaf.
For 2, I think I've not actually seen actual spam in the First Posts queue (but could be mistaken).
Examples can be provided (look for closed* posts in the First Posts queue, and the New Contributor notice in the Low Quality Posts queue), but I think people who have reviewed both queues would agree, and it's not really nice to link to people who aren't at their best.
*yes, I know not all closed posts are low quality.