If you look at our solar system, humans can survive on Earth, Mars, Venus and likely some larger moons without too much fictional technology. The universe seems to be full of rocky planets. The only insurmountable problem to living on a rocky planet with hydrogen, oxygen and carbon in some form really is gravity, if the planet is too big. But species that can travel interstellar distances must have the technology to live in space and only send robots to nearby planets or asteroids to mine raw materials. That increases the range of suitable systems considerably.

"Humans can survive on Mars"?

Check your priors.

They qualified that as “without too much fictional technology”. I think that is an important caveat.

What?

There is no evidence of humans surviving on Mars. Venus is more likely, but even Antarctica, thousands of times easier, only sustains visitors.

We have a pretty good idea of the environment on Mars and safe for potential long-term effects of the different gravity there there is no reason to believe that humans can't survive there using technology we already have, or could likely build.

Depends what qualifies as suitable. With enough engineering you could establish a colony practically anywhere with access to energy and matter. More near term, you could build self contained environments on or below the surface of planets and moons or in orbit. Longer term (slightly?), you could modify your biology to thrive in harsher conditions. Even further out, just bring your "planet" with you it would be a true wanderer then.

They may not need planets. They could build habitats in space.

Depending on the technological level of these societies, they may be able to terraform plants rather quickly