The Taenyong case had nothing to do with Choi Bit, that´s a different place. Taenyong was the place the main character Hwang was stationed at as a Prosecutor (iirc, it is the place he got sent to after season 1, meaning even after being part of the case against the President, he still went back). He was on his way to his farewell party since he got sent to Wonju next. Prosecutors below a certain rank have to change their station and place regularly (might be 2 years) to now get attached to the power structure of those places and thus to prevent corruption, which obviously works on a small scale but the big players obviously could not care less, considering most of them are in Seoul and have relationships with the high ranking Prosecutors who do not have to change their station anymore.
Basically: Hwang saw the Taenyong case coincidentally -> Former Judge and Lawyer Oh reached a settlement -> that case was done at that point.
But the key person that was tying all cases together was Seo Dong-Jae and with his disappearance, everyone was on edge and was suspicious of each other. And he was the one who was investigating the lawyer´s death Park Gwang-Su, the police suicide case and also noticed that the sole survivor of the Taenyong case, which he stumbled upon by accident, might have been a bullying victim and not actually a friend of the other two who died.
So you have
1.) Taenyong, the place the kids drowned and the rich couple were doing stupid stuff, also the place Hwang Si-Mok was stationed as prosecutor. This case was quickly closed because of the Chief Prosecutor Kang (who Hwang visits several times) and who resigns because of that, since his actions, even though he felt it was just a small favor to one of his sunbaes without any victims, led to Seo Dong-Jae getting kidnapped by the murderer of that case, and if that case had remained open, it might have prevented his kidnapping.
2) Sedok Police Station, which is a sub station of Dongducheon Police Station (bigger) (both not in Seoul but in the province surrounding Seoul). Sedok was where the police officer committed suicide, and Dongducheon was where the chief and uncle of Kim, police officer in Sedok, committed assault on his driver and thus was demoted.
3) Namyangju is also in the Gyeonggi Province, but a different place than the other two. The vacation home where Park Gwang-Su, who was working for Hanjo at the time, was gathering the people he wanted to recruit and a build a relationship with for Hanjo´s goals (for example the tax evasion case they were referring to several times) is there. The director of the Intelligence Bureau was one of the people gathered there when Park Gwang-Su died. He panicked and let one of his juniors handle the situation, which was Choi Bit, who was the chief of the Namyangju station, basically a small station. She saw an opportunity to help a high ranking police official and thus rise in ranks, and consequentially, she was promoted to Chief of the Intelligence Bureau right under that director. Then the director was arrested because of a different crime, but ultimately it was an attack by the Prosecutor Office to the police.
Besides Seoul, these are the three different places crucial to the story.
