They do it normally, by borrowing shares, for which you might find some figures.
They also do it without borrowing shares (naked short). If they do not cover quickly, then it appears on the regsho list. You will have a hard time knowing what volume it represents.
I think there are normally penalties for this kind of trespassing, but either it is not enforced or the fines are lower than the profit they make by using short selling.
This is my understanding at least, but I am certainly no expert in this field.
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