The first section, ["POST URL", "METHOD"], defines the header row. The rest of the query from the JSON data (in this case, a HAR file) is appended after the header row.

The deconstruction into a row of data happens at this section: [.url, .method]
This should match the order and size of the header row to get a proper CSV file.

The final selection, @csv is a special jq command that converts the list of arrays into a CSV format. The results are output to the terminal, but can be redirected to a file via stdout redirection.

jq -r '["POST URL", "METHOD"], (.log.entries[].request |
  select( .method == "GET" ) | [.url, .method]) | @csv' website.har