Hi,
It appears that you are using iMovie for iOS rather than iMovie for Mac. If so, read Paragraph 2F of the iOS iMovie license agreement at the link below. Your rights regarding use of the iMovie iOS audio content will be spelled out there.
https://images.apple.com/legal/sla/docs/imovieios.pdf
Paragraph 2F of the license agreement in part provides:
"Except as otherwise provided in this License or in the Apple
Software, you may use the Apple and third party content, including but not limited to, photographs,
images, graphics, artwork, audio, video, templates and/or similar assets (“Digital Materials”), contained
in or otherwise included with the Apple Software, on a royalty-free basis, to create your own original
video projects (and soundtracks may only be used for your own personal, non-commercial use). You
may broadcast and/or distribute your own projects that were created using the Digital Materials.
However, individual Digital Materials may not be commercially or otherwise distributed on a standalone
basis, nor may they be repackaged in whole or in part as clipart, stock animation, audio samples, sound
files or music beds. You may not use, extract or distribute, commercially or otherwise, on a standalone
basis, any Digital Materials outside the context of its intended use as part of the Apple Software. You
agree not to use the Apple Software, including any Digital Materials, for any unlawful or illegal activity."
Note that it grants usage on "a royalty free basis" as specified in the license, and does not say "copyright free". As stated earlier in the full paragraph 2F in the license agreement (not quoted above): "Title and intellectual property rights in and to any content displayed by or accessed through the Apple Software belong to the respective content owner."
So, it is possible that the "Illumination" soundtrack is copyright protected. If a copyright claim is made when publishing to You Tube the claim may outright bar use of the song, or specify certain conditions that would allow use of the song, such as no monetary use, or confinement to use within a certain country or geographical area, or agreeing to appearance of ads upon display of your video, or may require attribution. If you have no problem with the conditions, you can go ahead and use the song. Or, you can contest the claim as provided by You Tube.
Disclaimer: Nothing in this post is intended to be legal advice and cannot be relied upon as legal advice. For a definitive interpretation of your legal rights to usage of the software or its digital contents, or for interpretation of the license you should contact an intellectual property attorney.
-- Rich