Let's answer the last question first. When you use Insert Picture or Insert File to insert a file into a container field, you will see an option titled "store by reference". If this option is selected, FileMaker doesn't store the file anywhere, it just records the filepath to the file in the container field and fileMaker can use this stored filepath to find and open a file. If you do not select this option, a physical copy of the file is stored directly in the container field.
How you open such a file depends on that same "store by reference option" and also by the specific insert action used to insert it.
Insert Picture, store by reference: You can use export field contents to open the image file.
Insert Picture, don't store by reference: You can double click the container field to open the file.
Insert File, Store by reference: You can double click the file to open it.
Insert File, don't store by reference: You can use export field contents.
When using export field contents, you'll also need to select the "automatically open file" option.
Thank you very much! It's all clear now!