Sorry no I was referring to how when you shoot a slow-mo video on iPhone, when playing it on your phone you can adjust where the slow-mo starts and ends. See here for what that looks like:
This app may be worth checking out as well, for a nicer interface for adjusting video playback speed:
As indicated here, it’s a bit tricky working with slow-mo video in editing software. Default behavior is different depending which app you’re in, but for FCPX for example (or iMovie I believe) on import it will default to playing back at regular speed, and you have to edit the slow-mo parts manually. I think it’s just on the iPhone itself and in the Photos app on Mac where it has the draggable handles to set in / out for slow-mo as in the link above.
So for upload to YouTube, you could try setting a portion of the video slow-mo on iPhone and uploading directly from the phone, or import to a computer and edit e.g. duplicating the clip so you have it play twice, regular and slow.
One more thing to note — it sounds like slow-mo may not work if uploaded via the YouTube app, instead you may have to share to YouTube directly from the iPhone camera roll (as described in this video)!