Power Query Editor Transform file helper Query folders

Ingest = (Binary)=> Table.PromoteHeaders(Csv.Document(Binary), [PromoteAllScalars=true])

As you know each step in Power Query has a name.  You can refer to the step in subsequent steps by using that name.

Here we create a step that is arbitrarily called "Ingest".  Then we provide an argument  (Binary) which transforms that step to a function.  "=>" completes the function header.  After that you can have any number of steps that can eventually return someting.  In this case what is returned is the CSV interpretation of the function argument, with the headers already promoted.  We need the headers later so we can then combine/append the results correctly. 

Instead of nesting the instructions you can go through the whole let ... in ...  syntax if that's more relatable. And you can also place the function outside this code.  There are many ways to do this, and ultimately it is down to personal preference.