In most cases, the media you see in a Scalar book is not hosted within Scalar itself; instead, Scalar stores metadata — titles, descriptions, author and publisher information, URLs and more — about media which is hosted elsewhere, perhaps in a media archive, or a web service, or on a server you control. When we talk about "importing" media into Scalar, we're mostly talking about importing that metadata, not the actual media files. Further, once that metadata is imported into Scalar, you can modify or add to it as you see fit; these changes are completely local to Scalar and are never reconciled with the source archive. Conversely, if the source archive makes a change to its own metadata for a particular media file, that change will not be reflected in your Scalar book unless you reimport the media.
While it is possible to upload your own media to the Scalar server, individual files are limited to no larger than 2 MB in size. This approach has the advantage of making Scalar books very nimble, as they don't need to carry with them an extensive media infrastructure. However, it does impose certain limitations of which it is important to be aware.
Since Scalar books are hosted "in the cloud," a network connection is required to access them. However, since most of the media assets in a Scalar book are also hosted on servers maintained by other organizations or companies, if one of them has a technical problem it can render all media from that source within your book inaccessible until the issue is resolved. This is an extremely rare occurrence, but important to keep in mind while planning and troubleshooting.