For understanding 알쏭달쏭, examples would be helpful. Basically it means situations which are not clear, confusing and you don't know well. So for example, 'They talk vaguely' 그들은 알쏭달쏭하게 말한다. 'It was a real puzzling question ' 그것은 매우 알쏭달쏭한 질문이였다.
And 뻥 is informal expression of lie and same as fib.