The model is the AI that builds your app. There are two places to set it. Your account default lives under Account → Build Settings, and each app can either follow that default or use a model of its own, set in the app's settings. New apps follow your account default, so changing it moves everything that hasn't been set separately.
The list shows the models your plan includes. Next to each one you'll see a badge like 5x tokens. That's how much of your allowance it spends compared to the cheapest option, so a 5x model gets you roughly a fifth as many builds per day. Better models aren't simply better. You're spending tokens faster in exchange for stronger reasoning.
Use a more advanced model when the task is genuinely hard: building something substantial from scratch, restructuring how an app works, or a bug that's already survived a couple of attempts.
Stay on a cheaper one for everyday changes like wording, colors, spacing, or adding a straightforward page. You'll get far more done per day and the result is no worse.
If you're unsure, start cheap. Moving up after a disappointing result costs you one build. Running everything on the most expensive model costs you most of your day.
This is why per-app settings are useful: one ambitious project can sit on a stronger model while everything else follows a cheaper default.