ALL, you evaluate if all values in the list is true.
ALL([true, true, false])
This evaluates to false, since one of the values in the list is false.
In this example, we go over all the people in the current case and retrieve their value
'is_onboarded'. If all of these values are true, the whole expression is true. Thus, this evaluates true if everybody in the case has already been onboarded.