Several types of media are allowed including images, documents, audio files and content from external sources like YouTube, Vimeo, Twitter, Facebook, Instagram and the University of Iowa's Kaltura/Media Space service.
With all of these media types, you can add them from other content through fields and the WYSIWYG for display.
Each of the media types have specific documentation, but there are some shared functionality to highlight.
You can either add them directly to your site by going to Content > Media (/admin/content/media), or as you create content through fields and WYSIWYG text editors.
With replace file, you can go into an existing image, file or audio file and replace it with a new file uploaded from your computer. This will update any content where the media has been placed/referenced. This is easier than updating the file manually in each location it is used. By default, with the "Overwrite existing" checkbox checked, you will not only update all existing references to the media entity from other content, but you will overwrite the existing file and use the same directory and filename (e.g. ../2020-09/filename.pdf).