MUI (and Joy UI) is a lot different than a utility CSS framework. It has <Box>, which is a utility CSS component, and sx, which is a theme-aware way of doing inline styles.

The difference is that it comes with a <Button> component (and of course more), which has different variants, and you can set default props and default styles at the theme level.

So where an outlined button in a utility CSS world is class="... ... ... ...", or figuring out a way to be able to express that with class="button button-outlined", in MUI and Joy it's <Button>