Though the video wall is a fairly well used effect, it appears that the majority of the screens in that tiled wall section of your video, are stills. Looking closely, there are only a few that are actually moving video.
Ben will probably chime in here with the voice of experience, but aside from a plugin, I think you would need to do that in Motion to create a screen large enough that you can pull back from like that. Make your patchwork of stills & have 5 or 6 that are actually video. I think this video only has 3 or 4. They're also editing the sound so they don't overlap.
If you want to spend money instead of time
SugarFX makes a Video Wall plugin.
If you have the time & want to learn more about mastering these programs Motion VFX says
theirs is a "project" but you still have to pay for it. (perhaps they've confused the words project & product)
Doing some searches for "FCPX Video Wall" will lead you in the right direction. However, some of the solutions
like this one on YouTube are making a wall of squares where it's either the same image replicated in all the squares or one big image divided by squares.
Perhaps with the replicator idea, you could make an entire wall of multiple screens showing the same image, so the wall is a set size. Then black out or mask many of those squares so only 1 or 2 show thru. Then repeat that process with several more walls, but have the blacked squares act as alpha channels so the overlayed video walls could show thru on those squares. This way you could have your 10 walls making 20 squares with each video showing twice somewhere on the grid. All the walls would be the same size so you can keyframe them identically for the scaled zoom out. Stagger your duplicated images & the human eye probably won't pick up on it if they're spaced randomly & you don't let them look at it for too long.
The wall in your video, they only let us see it for about 5 seconds.
The extreme closeups on her mouth with that odd hue was an appalling decision.
