"Assets" are files such as images, PDF, Word or Excel files. You manage these files in the "Assets" area.
Think about a good filing order (folder structure, naming rules). You can also adapt these later. Usually the page structure is taken as a model.
After filing in the Assets section, place assets on a page - either with a download link (Link to DAM document) or as an image in a text/image component.
You can easily replace existing assets. To do this, open the asset and click the "Upload New File" button in the "Media" field.
It is better not to place videos in Magnolia. Playing them requires special technical features for which Magnolia is not optimal. See the recommendation in the "Video Player" user guide.