Before the loop in the 2nd microflow you should add a line with the column headers. just create 1 seperate object and put it to write next time before you go into the loop.
The first row (line) you need to create yourself. Here you can add all the collumn names. So before looping over the Iterator API you can create a first line outside the loop that creates all the collumn names (the first object of API_CVS)
By adding the create object and adding next line column is created but it is empty
You could add Header details to NewAPI and append this to API List before loop (Using Change list Activity),
The question would be is where does it add to head or tail,
If you have some Seq for each records you could determine where to add it.