The .txt suffix usually follows the Netscape cookie format , which uses tab-separated values. Which perspective are you interested in exploring further? Cookie-Editor extension - Opera add-ons
How the script locates the Opera GX "Network" folder to find the Cookies SQLite database. quickcookie-OperaGX.txt
These files are usually stored in AppData\Roaming\Opera Software\Opera GX Stable . quickcookie-OperaGX.txt
If you are trying to use a legitimate tool to your own data, the write-up would focus on: quickcookie-OperaGX.txt
Using tools like Cookie-Editor to export cookies into the Netscape/TXT format required by other applications.