AI Models
The message box has a model picker. Leave it on Auto and AppX chooses for you; every plan gets the same models.
When to change it
Most people never need to. Auto is the default, and it is the right choice unless you have a reason to pick a specific model for a message.
If a model you picked earlier is no longer offered, AppX switches the picker back to Auto.
Better results come from better prompts
The model matters less than the description you give it. A clear prompt that names the screens, the data, and the look gets you closer on the first build than any model choice.