Sharing large iPhone 15+ video thru iMovie 4k+60 FPS

Three questions: I have recorded several 4k+60 FPS and want to combine them in iMovie using iPhone 15 Pro Max. I see that when exporting the project to a video file, I must choose between 4k @30FPS or 1080p @60FPS. I don’t understand… how can I export movie file with the quality I recorded the videos (4k@60FPS)? Do I need to use a different app other iMovie?

Follow up question: if there isn’t a way around the limitation with iMovie or another app, if I just record video with desired quality and instead of stopping the video I simply pause recording and then resume for the entire project, making it one file instead of several, will I then be able to share the video online to platforms like YouTube with full quality or will it be reduced to 30FPs or 1080p?

Last question! If the sharing limitation is true, then how is Apple able to record all those high quality videos for the latest iPhone ads using only an iPhone?