If you don’t mixdown your audio, it’ll get mixed down when you export. Supposedly. There have been bizarre problems in the past. There have also been problems with the mixdown not letting go when you make changes AFTER a mixdown. Things like volume levels refuse to change audibly, even though you’re changing the levels. Have to delete the audio renders at that point.
If you don’t render your video it’ll get rendered when you export. A self contained movie simply means the RENDERED frames of video get copied to their own quicktime. No re-compression. But layers of video and effects have to be compressed (rendered) into one steam of video. If you don’t render before export, then it will render for you as I said, however it will just be to a cache file somewhere and your timeline will remain un-rendered when the export is complete. Might as well render it once in the timeline.