Congrats to the OP on an excellent accomplishment. I can tell you the system in my country (Singapore) since I was involved in it around 15 years back.
The pool of students to be selected from comprises those in Junior College (aged 16-18) who are preparing for their GCE Advanced Level exams. The best Physics students sit for a paper administered by their own schools. In my school, the paper comprised Special Paper questions selected for unusual difficulty. The Special Paper, BTW, is an additional distinction paper that is taken by the best GCE students who hope to win scholarships. The questions are fairly tough with heavy emphasis on applied calculus, but they are not Olympiad standard. The selection paper for the Olympiad uses the toughest S-paper questions for the initial round.
Following this, the top scorers are chosen to be part of the National Olympiad team. At this point, no formal training is given. The National Olympaid comprises a theory and a practical examination, you have to pass the theory before being allowed to take the practical. These examinations are of International Olympiad standard.
After the National Olympiad, there is a prize award ceremony and a selection of the top scorers for the International delegation hopefuls. Then the training begins with intensive sessions with University lecturers to whip the team into shape.
There is a final selection examination with tough Olympiad level questions before the final international team is chosen. They fly off to wherever, participate and have fun.
I went through the process and was selected for and participated in the National Physics Olympiad 1991. Unfortunately, I got a nasty virus and a high fever on the day of the examination and didn't do well, failing to qualify for the international team.

Still, it was a fun experience, just wish that I could've been in good health for the exam (after I recovered, I found the questions doable).