How do I transfer the thunderbird calendar to a new computer (win 10 to win 11, as well)

Can I assume you have access to both computers? And that is this a local calendar, not a network calendar like one hosted by Google or other? If so... In the source computer's calendar list, click the little three-dot indicator to the right of the calendar name, and choose Export Calendar and export it to an ICS file. Move that file to the destination computer. Choose the menu item (on Windows) <Tools | Import | Import from a file | Continue button | Import Calendars | Continue button> and navigate to your ICS file and choose it. Thunderbird will prompt you through the rest of the process.