Main problem here are the people who are working abroad as their main source of income. If they cannot go back to their work abroad, then that's going to be a big big issue.
People who are working abroad are allowed to travel, but it's quite difficult without plane flights.
Though, my friend travelled to Germany by feryy, then by car to Netherlands. And he didn't even had to spend 14 in quarantine - he started to work next day after arrival.
What about the import and export? If borders are closed then imagine the life living in a country like North Korea.
Obviously, borders aren't closed for important and export - it would cause complete collapse of economy which is already hurt seriously. Trucks with good are moving as usual between countries as usual.
Closed borders now mainly affect tourism sector - airline companies, tourism agencies, hotels and etc.