How to export civievent participant list to a csv file?
What version of Civi are you using?

Under Events > Reports > Participant List, Mine shows "Export CSV" as an Action Option

Under Search > Find Participants, it just says "Export List", But when I export it I get a CSV file.

But even if you get a .xls or other Excel file you can always save it as a .csv in Excel. If you do not have Excel, then run it through an online file converter. You can even open the Excel file in Google Sheets and then download it as a csv to convert it.