I created a Google Sheet. It has one worksheet. I changed URL so it has export instead of edit and set ?format=csv. Here is the final URL:
https://docs.google.com/spreadsheets/d/{spreadsheet_id}/export?gid=0#gid=0?format=csv
It returns .xlsx file.
What am I doing wrong?
There are two instances of ? there. Use just one, like this:
https://docs.google.com/spreadsheets/d/{ss_id}/export?format=csv
You can also specify a sheet other than the first (default) sheet with &gid=..., like this:
https://docs.google.com/spreadsheets/d/{ss_id}/export?format=csv&gid=123456
To refer to a sheet by sheet name rather than by sheet id, or to refer to a specific range, use a query string:
https://docs.google.com/spreadsheets/d/{ss_id}/gviz/tq?tqx=out:csv&sheet=Sheet1
https://docs.google.com/spreadsheets/d/{ss_id}/gviz/tq?tqx=out:csv&range=B2:D42
Query strings also let you filter and aggregate data.
Also see Download and export files.