The first one looks like as the camera passes under the bridge, a key was applied to the blue sky to make it transparent. Then the next clip was layered underneath to make it show through the key and gradually appear as the camera emerged from under the bridge.
There are probably basic tutorials on YouTube, etc. on keying in FCPX. I think Larry Jordan did a free one about adjusting a key. It's really the same as doing green screen, except you choose a color other than green to key out using the Sample Color adjustment in the Keyer. It looked in the video like the sky was a fairly evenly colored deep blue, so that made the keying possible.
1, Option-drag the clip to make a duplicate on top of itself.
2, Apply the Mask effect to the lower copy, and adjust to mask across the center of the bridge, and everything above it, to hide that portion in the bottom clip.
3, Apply the Keyer effect to the top clip. Click the Refine Key icon, click and drag across the sky until it is keyed out.
4, Place background clip below the two duplicates.
If it's not working, then you aren't following Ben's steps above.
We may be assuming more FCPX experience than you have
The previous, outgoing clip has to be above the next, incoming clip in the Timeline
Drag the Keyer (from the Effects tab) to the upper clip
It will by default key out lime green. You have to tell it to do something different. The way you do that is to highlight the clip you dragged the Keyer onto, then in the Inspector you should see Keyer at the very top under the word Effects.
Just below the word Keyer you will see the words Refine Key: and just to the right there will be two thumbnail images.
The left one is Sample Color and shows a little selection box beside a person's profile. Click on that thumbnail, then in the main screen drag diagonally to select a piece of the sky (or whatever color you're wanting to make transparent).
Depending on how consistent the sky color is, you may have to tweak the selection, and depending on the outgoing clip's look you may want to crop or mask parts of it (as Ben showed above) to get the look you want. And you may need to experiment with the Strength, Fill Holes, etc. options in the Keyer, but the starting point of keying out the sky is very straightforward.
The attachment shows a street and a house, but where the sky should be is some guys on a golf course.