For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
Please see our announcement here.
But as I said, export creates a zipped XML file, and import on the list page accept only CSV files.This will create a file form-name.xml with the structure of your list/form (groups, elements, validations...) stored in your-site/administrator/components/com_fabrik/models/content_types
Create a new list, set label and table name, Save
Select the content type you just have created, Save+Close
No, content type is for new tables only.I select an existing table.