All files in your Google Drive online and Chromebook offline files may be found using the Google Files App. You can find this app on your Chromebook Launcher near the bottom of your screen.
HOTKEY: Press ALT + SHIFT + M at the same time to open the File app on your Chromebook.
This video is an excellent tutorial for using the File app. It highlights how to find online and offline files. You can also see how to use USB thumb drives.