The media files loaded into the Resources window are called assets and are virtual references to the real media files on disk. The original sources are not changed and so we have non-destructive editing.
There are many supported file formats that can be loaded and rendered to, that is to say, imported and exported. The format of the file affects what CINELERRA-GG does with it. Some file formats are very slow to display on the timeline, especially video which is highly compressed. Drawing video thumbnails, picons, on the timeline can be very slow. (You can disable picon drawing for these files with the draw media toggle in the patchbay to speed up operations).