Example one:
Once they give their money, that's it, they don't want to hear it anymore.
Once they give their money, that's that, they don't want to hear it anymore.
Example two:
Today's lesson will only cover the introduction about the course. And That's it
Today's lesson will only cover the introduction about the course. And That's that
Are the above examples with That's that correct? I feel that That's it and That's that has a little difference in connotation and That's it has a larger arrays of meaning and can be used in more situations that the other, is that true?