THANKS!It’s the pop rivet that holds the roof side trim moulding:
The mandrel of the pop-rivet is designed to break off after the rivet expands. Rivets used to be the most popular method of fastening aluminum components. Pop-rivets are used for non critical fastening and when there’s only access to one side of the panel. The accepted method of removal is drilling them out, which is a little easier for pop-rivets but still not fun.