Hi,
The answer is that the area of digital rights and copyright is a quagmire.
You can read the current license applicable to your version of iMovie by opening iMovie and clicking on iMovie/About iMovie/License agreement. You can decide for yourself, or consult a lawyer about, what your rights are to publish on social media your movies that contain audio and video provided by the iMovie software. Paragraphs 2F and 2G appear to be the most relevant ones.
Paragraph 2G states in part: "Except as otherwise provided, all Sample Content included in the Apple Software may be used on a royalty-free basis in your own video projects but may not be distributed on a standalone basis."
I would think that "Sample Content" includes all those items listed under the Audio/Sound Effects tab, the jingles, etc, but that's just my personal non-legal opinion. The agreement doesn't define "Sample Content".
You Tube will advise you of copyright claims that are made regarding use of music in your video. Sometimes the claimant will consent to unrestricted use. But publishing on You Tube will not allow you to "get around" any license violation, if any exists.
As I said . . . it is a quagmire.
-- Rich