Welcome to Filemaker; enjoy the learning. To see what is going on in the Assets starter file you need to go into Layout mode and see what the layout comprises—you will see that along the bottom there are five separate container fields; above that is another larger field. The smaller fields also have a script activated by a script trigger to post the field contents into the larger field. To dig deeper you need to study the database structure (File –> Manage –> Database) and also the scripts (Scripts –> Manage Scripts).
A word of warning though. The method used in this starter solution is pretty elementary and will break down as soon as you have a record in which you have six images to store. So you should start thinking/learning about other techniques. A much better method would be to create a separate table for container data and link its records to the relevant parent record; this way you can have an infinite number of images associated with any parent record.
That is just a couple of starting thoughts. Keep thinking, keep reading, keep asking questions.