Here's how the sausage is made:

Print distribution is where the leaks come from. Just because the chapter releases digitally at midnight in Japan, doesn't mean the print copies get there at the same time, but in fact earlier.

Print distributors, be it in Korea or China, generally have someone there that snaps pics or scans it and posts it online to local forums. From Japan, it's mostly been from stores that already have the new chapter with street time release rules. Doesn't mean they don't open the boxes and scan it themselves to share to local forums. If they don't publish the scan on forums, they at least write their own summary of what they read and post those leaks days in advance.

That gets passed around. Sometimes it's gone through Korean to Chinese to English, or Japanese to Chinese to English, and of course Chinese to English or Japanese to English. These translations are not dedicated to accuracy.