It doesn't. The Martian environment sets the limits on everything else. You could drag the entire Earth over next to Mars, and people still couldn't live there.
The irony is that the hypothetical "What if the Earth became uninhabitable?" is effectively the same as "What if the Earth became just like Mars?"
Not in the slightest. SpaceX's proposed Starship design has known mass and envelope limits. All of the required support systems can be designed with those limits (or subset thereof) as part of the constraints. In fact now is the best time to start designing those because it can inform design criteria or mission profiles for Starship. Having those as handwave-y unknowns while talking about colonies is just absurd.
A manned landing on Mars requires they have months worth of reliable support infrastructure available. None of that is just going to appear. It all needs to be built and landed with or before humans. It needs to be repairable with tools on hand. It must power on an be functional on Day 1. You're not serious about landing people on Mars let alone building a colony without talking about the "boring" infrastructure that will keep everyone alive.
Hell, as it stands it isn't even clear if initial vehicles will need auxiliary thrusters to avoid digging up a hole upon landing. Even NASA seems to be uncertain about that one.
So yes, it is ridiculous to argue that they aren't serious about setting up a colony based on what they're doing right now.
Beyond that though, they're working on the spacesuits (granted, they're so early into that they're nowhere near ones usable on Mars) and they have demonstrated the ability to perform some amount of important maintenance on the Starship vehicles without needing machinery that isn't easy to bring along.