Chromebooks utilize the Chrome Operating System and save the majority of files in the cloud. You can save to one of 3 locations:
1. The "Files" folder
2. Google Drive
3. External Flash Drive
The "Files" folder contains items that are saved locally but this is NOT recommended since Chrome OS's design is to use local storage as a cache. Meaning it will wipe local storage as it deems necessary without notice and is limited.
The preferred way to store items is to use Google Drive. You can set Drive, or a folder in Drive, as your default location to save files to.
If you are using a program that requires files to be stored more like a desktop environment similar to the Windows and Mac operating systems, then using an External Flash Drive is the best route as this will let you save and upload files to the respective software/application in this manner.