Yes iPhone sounds are copyrighted by Apple.
However I've never had any issue with using Iphone Sounds in any Youtube Videos that I've made.
But if an Apple Employee came across the video then they might make a copyright claim but so far Youtube does not detect the Iphone sounds as copyrighted.
Sound effects can be subject to copyright especially if they are modern sounds such as Iphone sounds.
It's best to look online to find public domain non copyrighted sound effects so you don't have to worry about your video getting a copyright claim.
If you do get a copyright claim it's best to remove the sound effect from the video or just use public domain sound effects.