1. Update your iPhone or iPad to iOS 12.
2. Download the Shortcuts app (it may already be there if you've used Workflows).
3. Open this page on your iPhone or iPad and tap Get Shortcut.
4. Tap the Share button in any app.
5. Swiping left on the bottom row of icons until you get to the More button, and tap the More button. In the YouTube app, you'll do this twice.
6. Then, turn on the switch next to Shortcuts.
7. While watching a YouTube video, tap Share
8. Select Shortcuts.
9. Tap Download YouTube.
10. Your file will download.
11. Tap Save to photo album.
12. Your video is now in the Photos app!
After you share that movie file to another system, one that recognizes files, you'll see that it's an MP4!