On SQL Developer on macOS click on the menu
Oracle SQL Developer > Preferences...
then expand
Database > Worksheet
then in the option
Select default path to look for scripts
enter your desider default path, for example
/Users/your-username/scripts-folder
Now, when you open the menu
File > Open
the first suggested location on the left will be the one you chose.
You can also display the Files pane clicking on the menu
View > Files
so you can easily access your chosen folder below the My Computer folder. (remember to click on the refresh button after you change your SQL Developer Preferences, so your folder appears below the MyComputer folder.