This area of the Editor is good for the preparation stage for when you are about to start creating your new publication. It's a good idea to gather all your files together in one place and name them accordingly so you know where everything is and what it's called, then your workflow speed will stay consistent.
To use the File Browser you will start off in the publication, then on the right side of the editor, you will click the Files tab. This tab shows the files that you have uploaded to the Media Library while editing this publication. From this tab, you can see the files uploaded for this publication, but you can also browse to another publication and see the files and use those files that have been used in a different publication. Now you will want to click the green Media Library button towards the top of the tab. A popup will appear, which is the Media Library, here you can upload new files that you want to use or organize these files into folders. You can use the search bar to file a specific file among other files and folders. On the right side of the popup, you will see a drop-down menu where you can select to see the files that have been used in another publication. Under that is the Information of the file you have selected in the folder or file navigation area(Center for files, left of the popup for folders). The top center there is a search bar where you can search for a specific file name. To the right of the search bar, there is two button that changes the view layout of the files, one for the block view of files, the other is list view of files. In the top left, the double down arrows show the options to make new folders, refresh or upload files. The arrows next to this are the folder navigation files if you want to go back from being in a folder or going to the folder above etc.
The Assets & File Browser will open when you add a file to an element. So, when you want to add a video the window shall open; when you want to add an image the window will open, and the same goes for audio files.