How to handle a loop end of column list loop

Hi everybody!

I have created a Column List Loop Start to deal with each column data.

At the end of the loop, the result of each loop need to be concatenated ( like to concatenate all columns to one column).

I cannot find how to handle the end of the loop, how to concatenate column of every loop into one column. Is it need to be write to a csv file in every loop?

If detail information is needed, please reply me and I will upload my data and current workflow tomorrow then.

Thank you!