One source? There are multiple sources, including eyewitness accounts, reports from Chinese and Uyghur exiles, western experts, and even satellite imagery:

"The findings of this research contradict Chinese officials' claims that all 'trainees' from so-called vocational training [centers] had 'graduated' by late 2019," report author Nathan Ruser wrote. "Instead, available evidence suggests that many extrajudicial detainees in Xinjiang's vast 're-education' network are now being formally charged and locked up in higher security facilities, including newly built or expanded prisons, or sent to walled factory compounds for coerced [labor] assignments."

https://www.npr.org/2020/09/24/916665128/report-satellite-im...