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
Using Developer Tools (Ctrl+Shift+I) in Opera GX to view and copy cookie values. quickcookie-OperaGX.txt
Using tools like Cookie-Editor to export cookies into the Netscape/TXT format required by other applications. quickcookie-OperaGX.txt
How it uses a tool (often called "QuickCookie") to decrypt and save these cookies into a .txt file for a hacker to bypass your logins. 2. Developer/User Utility Guide quickcookie-OperaGX.txt
How the script locates the Opera GX "Network" folder to find the Cookies SQLite database.