Some extra info-
Dialects:
They were often speaking in a different dialect (aka santoori). Some were imitating, but Yunho has a Gwangju/Jeolla accent. It sounds kind of bright compared to the Seoul dialect. Seonghwa and San have a southern Gyeongsang accent (Busan area). This satoori is used a lot in Korean gangster movies. You can hear this well when Seonghwa reads the letter at the beginning.
Also, they used phrases in their dialects in their track The Real.
Sparkling:
At the end with the sparklers, they were making references to the kdrama Imitation which starred Yunho, but also had Jongho (who along with his actor sidekick stole the show honestly), Seonghwa, and San.
Yunho, Seonghwa, and San were part of the pretend rookie group Sparkling in the drama. There was an awful fake song they did calling Diamond where Yunho sang the infamous “Finally Now! It’s my time!”. You’ll hear them clowning it A LOT.