You can make a compilation video on iPhone using iMovie (a built-in iOS tool) by following these steps:
Step 1: Start the iMovie app, and then touch Continue if you get the Welcome to iMovie screen. Create Project, then Movie
Step 2: Choose the videos you want to include in the compilation and click Create Movie.
Step 3: You may now view the video and add transitions between short clips by moving the clip to the left or right till you locate a cut point, then tapping the transition button to choose a transition style.
Step 4: When you're done, touch the Done button in the upper left corner of the screen. To submit the video to YouTube, tap the Share icon at the bottom of the screen and choose YouTube.