video at the ending credit?

I had the same question as you but after reading some reviews, i did understand what it was showing. I'll quote from those reviews instead of explaining it. Btw, that footage may have been also shown in the beginning of the movie instead of at the credits.

From a review:
"Pluto opens with an unforgettable footage—a look at a day when the College Scholastic Ability Test (CSAT), a standardized university entrance test which is held only once a year and lasts for nine hours, is underway. Parents are praying at temples, police officers and road blocks are everywhere, making sure that nothing will disturb the test. Apparently, all the adults’ working lives are put to a halt on that one morning each year to avoid traffic jams that could delay the students’ arrivals to testing sites. Even flights are not permitted during the test’s speaking portion."

And this is from another review:
"The film never lost my interest though it was truly frightening to see how the stress and expectations of society pushed these students to do horrible things. How bleak their world seemed even though they were only 17 (19 in Korean age); no loyalty, friendship driven by fear. The pressure for entrance exams and the opening scenes of people praying and lining the streets to cheer exam takers are real, with Korea having high student suicide rates in reality."

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:)