Humanity left Earth to escape Godzilla. In their relativity, they only spent 20 years away from the planet, while the planet didn't see them for ~20,000 years.
When they return to fight Godzilla (not a spoiler), you see large landing ships, hoverbikes, powered suits, and many other pieces of high technology. Their legged tanks are even strong enough for Godzilla to feel threatened by.
Did the technology of the people who left advance that far in the 20 years they were gone? Or did the other 2 alien species impart their technology to the survivors?