By default - an App does.
However, you have to choose the type ahead of time and this will dictate the screen sizes.
Choose tablet an it will have good results on desktop/laptop/tablet. On a phone, it will be in landscape mode only and can be small depending on how you size your objects.
Choose mobile and it will look good on mobile, but seem limited on desktop/laptop/etc.
You can't create both on the same logic. You would have to incorporate the logic into two apps - which can be a versioning nightmare.