Hey there! Thanks for reaching out! Premiere on iPhone exports videos as MOV files (using HEVC/H.265 codec), not MP4. This is Apple's native format and generally provides better quality and smaller file sizes.
Is there a specific issue you're running into? Do you need MP4 specifically for a certain platform or use?
Let me know more about what you're trying to do, and I can help troubleshoot or suggest the best approach!