How can I set this as my default date format?
How can I defualt to everytime I create new spreadsheet line 1 is always highlighted, in bold and has a filter feature?
Excel takes default format Windows setting (if you are on Windows). To change it Win+R, type control international, Ok and change Short Date format.
Not sure about second question, you may play with Excel default template https://exceljet.net/blog/how-to-set-a-default-template-in-excel