Ableton Live projects are stored in folders, containing all pertinent audio files, processed files, MIDI, and other project details. When files are added during mixing (drag-and-drop, etc), their location on the disk is referenced BUT the file itself is not automatically copied into the project folder.
In order to make sure that all files are stored together in the project folder (to bring to another studio, to upload as part of an assignment or finished project, etc), please use “File–Collect All and Save”, which gathers all files into the project folder automatically.
See here for more details: How to ‘Collect All and Save’ a Live Set
If you wish to view a more detailed view of all gathered sounds in any project, this can be done with the “File Manager”, with more information available here.