Hi Martina,
The “device-width” entry will scale your prototype to the width that you designed it in Axure RP. If you designed your page at a width of 375px, for example, then having “device-width” will make sure that the page is viewed at this size. You can read more about this here.
If you want the page to automatically scale to different screens/devices, you’re on the right track of using adaptive views. Adaptive views allow you to design your prototype for multiple screens:
https://www.axure.com/support/reference/adaptive-views
If you have a general idea of the most commonly-used screen sizes, this should help you to get close to having a responsive prototype. An alternative would be to add an OnWindowResize event to resize all widgets on your page to [[window.width]], but keep in mind that this will take a hit on load performance, depending on how many widgets are on your page.
Lmk if you have any questions. 