In Notepad Plus Plus, when you have a list of strings and you want to have one line with these strings separated by a comma, you can use the Replace function.
Let’s assume you have this list
and you want this outcome:
Apple, Orange, Strawberry, Banana, Kiwi
Then do this: CTRL + H to open the Replace window. Then select Regular Expression in Search Mode. In the Find What, enter [\r\n]+. Then in the Replace with, enter the comma (,) and maybe followed by a space if you also want to add that.