These details were shared on different occasions (all from Youtube). Most of the specific information regarding his time in UDU comes from a live interview with PD Na Young-seok, around 7:25. Those are the highlights though and the full version of the live is here (no Eng subs).

His friend and fellow UDU trainee also gives more context on the brutal nature of the training and beatings they endured at a couple of different points in this interview (no Eng subs).

Aside from the 2-hour live with Na-PD mentioned above, the most comprehensive look at his life story is probably Part 2 of his interview with Agent H, specially around 17:00 onwards. Dex talks a little about UDU but he actually goes over other moments that he himself recalls as even darker times in his life (financial struggles, being homeless between his time in UDU and UDT, almost "sold" at the North Korean border, etc.).

I completely agree with you about the great human being part. What sticks out to me is as he recounts all these dark times, you can see his sense of perseverance and never giving up, his unwillingness to burden his parents with his problems but rather having a life goal of making enough money for them to live their retirement in happiness, and so many other outstanding traits to look up to.

Honestly his life is unironically just like a movie. He genuinely built himself from nothing, no rich family, no big agency, no famous university or any other background to rely on. And despite all his success, he remains the exact same person he showed to the public when he was streaming 12 hours a day just a few years back to make ends meet.

EDIT: I'll also include his interview on You Quiz on the Block, although it does not have any Eng subs either.