The File Explorer has 3 different display modes, accessible at the top right corner
The navigation in the subfolders is done by unfolding them, by double-clicking their name.
To access files inside subfolders, you still have to double-click them, but this time you enter the folder and only see its content. To get back to upper directories, you can use the upper bar, representing the path to you file, by clicking the name of the destination folder. The "Home" icon represents the root of you drive.
The "Icons" mode allows you to quickly view images and videos with associated proxies. Simply move your mouse from left to right over a thumbnail to preview the content of a video. This navigation mode is similar to the "List" mode.