The sequence shows Kaijus decimating Manila, San Francisco and Los Angeles. And it had to sell the believability of this world, right off the bat. As Mirada's VFX supervisor Zach Tucker explains:

The prologue montage covers a lot of ground in a short window so we had a lot of information and visuals to pack in. Our work also had to look believable and match the quality of ILM’s work. To achieve this, we didn’t take any shortcuts and built a ton of detailed assets from scratch, from Kaiju to Jaegars and more, with all the animation, lighting and compositing done in house.

Check out the making of each part of the sequence:

Monster Face

Robot Arm

Jaeger Hangar

Crashed Jaeger

Jaeger Attack

Standing Together