Is there a Cole's notes for the baseball world classic? aka what the heck?

I keep seeing this on tv, had assumed it was a junior tournament but then saw Naylor playing and he's one of the handful of baseball folks I know. So unless things have gone really badly for him over the summer...

How do teams decide "yeah, they can play in that" versus "they need to play our spring training" ? Also, why the heck is Puerto Rico its own team?

I tried some googling but didn't get much there. Cheers yall!