How can use the export option from file menu to export several files automatically, Should I use command line tool for this action?
One way is to use the command line. There you can use gptor pconvert. You can find both in the bin folder of the SNAP installation directory.
You can type gpt -h or pconvert -h to get more help for each tool.
Also possible is to use Batch Processing in SNAP Desktop. You can find in the menu under Tools.