SpaceX created the first fully reusable rocket stage and, much more importantly, made the reuse economically viable.

Making life multiplanetary is fundamentally a cost per ton to Mars problem.

It currently costs about a billion dollars per ton of useful payload to the surface of Mars. That needs to be improved to $100k/ton to build a self-sustaining city there, so the technology needs to be 10,000 times better. Extremely difficult, but not impossible.