How is the Aggregate Transformation different from the Expression Transformation?

Added again.

In the Aggregate Transformation, you can apply different aggregate functions such as Sum, Count, First, Last, Min, Max, Average, Var or StDev and derive values from the input.

Whereas in Expression transformation, you can apply various functions such as string, date/time, financial, matching, logical, mathematical, regular expressions etc. on your incoming data or even write your own expressions in the expression layout builder.