After you have started an iMovie for iOS project and have added video clips to your timeline, you can adjust the look of each clip using filters and also by combining clips in picture-in-picture and split-screen arrangements.
To add a filter, tap on a clip in the project timeline to select it. With the clip highlighted with the yellow border, tap the filter button -- it's the right-most button of the five in the lower-left corner and features three overlapping circles. You will see 10 Instagram-like filters from which to choose. After selecting a filter, tap the Play button on the right edge to return to the timeline view of your project and watch a preview of the filter you just added.
You can add a different filter to each clip of your video. You can also add a different filter to each clip in picture-in-picture and split-screen layouts by highlighting each clip in the timeline and then applying a filter.
Moving in the opposite direction, you can also add a filter to an entire project. To do so, tap the gear icon that sits to the right of the preview window and then choose a Project Filter.
I should back up a bit here and explain how to create the picture-in-picture and split-screen overlay effects, along with a third overlay effect, the cutaway. Additionally, you can add just the audio of a video clip. All four of these options are available when you are selecting a video to add to the timeline.
When you select a video clip to add to your project, you will see five other buttons to the right of the down-arrow button, which simply adds your selected clip to your project. Let's tackle these five left to right. The first is a Play button that lets you preview your clip before adding it. The second button shows an icon of sound waves and adds only the audio of the selected clip. The third button adds a cutaway clip so that your video will cut away to another clip while continuing to play the audio of the underlying clip. The fourth button adds the selected clip in a small picture-in-picture window. Lastly, the fifth button adds your selected clip next to the first clip, and of equal size.
For any of the above effects, you must drag the timeline so the white vertical line in the middle of the timeline is positioned at the point where you want the effect to begin. You can also trim the start and end points of any clip in your timeline by highlighting it and dragging the yellow bars at the left and right ends of the clip.
After adding an overlay clip, you can edit it or change it to another type of overlay effect altogether. In the timeline, an overlay clip sits above the regular clip in the timeline. Tap on an overlay clip and you'll notice the T button to add title graphics changes to a two-rectangle icon. Tap on it and you'll see the three overlay options for cutaway, PIP and side-by-side, plus three other side-by-side arrangements that include top-and-bottom split screen in addition to left-right and right-left split-screen options. For a PIP overlay, a second button gets added above the zoom control button in the lower-right corner of the preview window; tap on it to reposition or resize the PIP window.
You can slow down or speed up a clip. Highlight a clip and tap the speed dial button next to the scissors button. You will now see a slider. When you adjust the speed, a yellow bar appears in the timeline to show you the duration of your speed change; you can drag the ends of this yellow bar to adjust the start and end points of your speed adjustment. You can also add a freeze-frame effect by selecting a frame in the timeline and tapping the Freeze button. You can then drag the start and end points of your freeze frame in the timeline to adjust its duration.
There is one setting related to changing speeds in Settings. Tap the gear icon and scroll down to the last option listed: Speed changes pitch. With this setting enabled, the pitch of the audio sounds higher when you speed up the pace and sounds lower during slo-mo scenes. Think chipmunk voices during a sped-up scene. With this setting disabled, audio is sped up to match the pace of the video but the pitch remains the same as at regular speed.
For more, learn how to add a title sequence in iMovie for iOS.