You've been given very good evidence what their base salaries are. Every extra shift on top of that is overtime. Like in most jobs people don't work extra for free. 36 hours is very normal.
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I should be asking you for evidence they work way more hours for their base, as it is your claim that is less likely to be true.
There are also lots of articles on this if you really bothered. But I suspect you'd want them to print a contract, as "they must be working lots of hours" is your starting point.