Click on the three dots in front of the file's title.
Click on Copy.
Choose the destination project or sub-project.
Click on Copy to confirm or Close to cancel.
Now you can find a copy of that file in the project destination.
To copy several files:
Select multiple files.
Right click on any of those files.
Choose Copy Selected Files.
Choose the destination project or sub-project.
Click on Copy to confirm or Close to cancel.