
Guyss I found it, another clue at chapter 24 when Myeong-il looks through the logs, each one ends with "Tape sent as of [date]" so the prison sends those videos to the Uncle.
So, the tapes the creepy Uncle has in his home, are of Jingi torturing the other inmates AND of when Jingi attempted su*cide multiple times when he first got imprisoned.

Damn ngl, if my fake parents were already sh*t, AND my real parents were insensitive enough to ask me to help the bish who tortured me. I'd just cut off all the parents.
Like, mom seriously? If you wanna help your former daughter then help her yourself, but can't you be more sensible and try not to involve me anymore??

Re-reading and I just realized... who visited Jingi at chapter 9? At first, I thought Jingi saying "I don't wanna see his face" was referring to the Uncle.
But they meet him at chapter 18, and they talk like they're catching up and haven't met for a long time. So, it's not the Uncle.
Maybe Jingi and Heonjae's father? No. Jingi seems to hate him based on what Heonjae said in chapter 20.
Could it be... THEIR THIRD BROTHER?? (mentioned in chapter 27). Who else would visit them other than family?
Idk why their Mom didn't visit before since she's probably already been released from prison a long time ago...
Ah, could it be the possibly middle or oldest brother was the one taking care of their mother while Jingi and Heonjae was in prison?
Who was Jingi using his phone calls for in chapter 29? It has to be the other brother right? There's no other family left to call.
But wait, why didn't the brother inform Jingi about their Mom dying? Why didn't Jingi want to see his face? Bad blood? why? Hmm, damn...idk.
Also, why did Jingi's mom ask for a picture of Jingi to be included in the mail? (chapter 30). Does she know she won't be able to get out of prison for a long time and won't get to see him often, so she wants his photo while she's in prison?
There's a lot of clues lying around this manhwa, that tells a story not being told directly or outright, but in small details that are told quietly like whispers of a secret we aren't in on. Especially in the first 20 chapters.
If you read it again, you'll see there's a lot of panels and dialogue that you probably missed noticing in the first read.

Purple hair bish is an example of forced plot problem creator. She couldn't text him because "she couldn't think of anything else that night"?? Wth. It takes 5 seconds to text someone you just ruined their relationship.
What'd you do all night and the next morning???? And the morning after that??? You couldn't text him then either???? Whatt? It took about 2-3 days after the lying incident before she set up this little meeting.
Which AGAIN could've just been a faster text than going to a cafe with a drink. I thought you was scared of the manager seeing you two together, so why do you keep asking for useless meetups all the time??
An email, a note, or through the other co-workers. Anything could've been possible to relay such an important thing you just did. Telling someone's girlfriend you've been cheating with their boyfriend seems VERY important to remember.
Nahhh they're gonna think he's dating that manager lady who takes the front seat in a car of a co-worker who clearly distances themselves from her and wears hair ties that she knows belongs to their girlfriend.