It's an and, not an or. People get good at living in space. Some of them move next to a new planet because it has resources. Some of those people have to go to

It's an and, not an or. People get good at living in space. Some of them move next to a new planet because it has resources. Some of those people have to go to the planet to exploit those resources. The rest stay in space. Now you've got a populated planet. Maybe they'll do some terraforming. I'm not sure what will be practical, but it's not hard to imagine that at least some basic terraforming is probably feasible just by constructing life forms that can live on the planet and further move it in a direction intelligence likes, even ignoring massive, energy-intensive interventions. Not all the space people have to move to the planet, just like when North America was populated it didn't depopulate where they came from.