At San Juan Luis Muñoz Marín airport, as at every airport in the United States, passengers arriving on an international flight must clear immigration (CBP) and customs, collect their checked bags and re-check them, and go through TSA security again before their connecting flight, even when both flights are on the same ticket.
The airport has 2 terminals or concourses (Terminal A (JetBlue and others), Terminal B (Concourses B, C, D)).