To view the contents of your external hard drive, you need to locate the Files app. If you have not rearranged the icons, it typically is in the dock at the bottom of the screen (look for the blue folder with a white background it should look like the image below, as well as a link for more)
If you cannot find the folder, from the home page, swipe down from the middle of the screen and search "Files". Or, if you can't find it that way, you can ask Siri to "Open Files"
Once there, you need to click on Browse on the bottom and expand "Locations". Look for the logo in the red box in the second image to locate your drive (or if you know the name of it, look for the name.
All your files on the drive will then appear. You can browse them now!
Please see attachments below if you need help importing or exporting photos/videos.