Looking at the IF formula, the formula starts with a condition, [MonthNo] < 4, for the first parameter. If this condition is true, then the next parameter is returned, in this case Q3. However, if the statement is not true, then another IF statement is evaluated and the pattern continues. It is important to understand that IF conditions are evaluated in order. In other words, the first IF statement is evaluated first and must be evaluated as either true or false before either returning a value or executing the next IF statement. This is why we do not need to worry about month numbers 1, 2, and 3 when evaluating [MonthNo] < 7 in the second IF statement. We can be certain that if this expression evaluates to true, then [MonthNo] must be 4, 5, or 6, and not 1, 2, or 3, because these latter numbers have already been excluded.
For the SWITCH statement, the first parameter is the value to evaluate, in this case [MonthNo]. The subsequent parameters come in pairs, the first value of this pair being the potential value of the expression in the first parameter, and the second being the value to return if the expression in the first parameter equals the paired value. At the end is a catch-all else value, the value to return if no other conditions are met. As you can see, the SWITCH statement, while more verbose, is much easier to read and understand.