They deposit the coins into organisations that mix coins from various customers. Its just like a bank. You deposit a fake/stollen 100$ bill one day and, if they accept it, you can withdraw a totally different 100$ bill from an ATM the next day.
I never actually understood the whole "bitcoin mixer" scheme. Shouldn't all all the transactions still be traceable in the end?
Is that something that is actually needed for something or just money "laundering"?
But then in turn I'm surprised that states haven't yet sanctioned the mixers themselves. I understand there's no proof that they might not also mix legitimate funds, but I wouldn't have expected this to be a big hindrance given that everyone seems to know that most of their use is for money laundering.