This is a small tribute to the main four characters that kickstarted the whole Marvel Cinematic Universe.
But then again, without the comics, those movies would never have existed. The animation and look is designed to closely resemble the old comic versions of the characters rather than their movie counterparts.
The animation loopdeloop is based on each character's “Hero” pose. The Captain America shield block and shield throw. The Hulk's rage transformation and battle roar. The Thor lightning leap smash and Iron mans flying and repulsor blast.
It is all animated and drawn frame by frame in Photoshop. Then composited in After Effects to get that polka dot old-time comic book style. The sounds are all gathered from the MCU movies themselves, and then mixed together.
Watch the process below: