Formatting across multiple devices

So as I started designing my app (which is going to be used for my dissertation) I started to wonder about the usage of the app from different devices. If someone accesses my app from a desktop computer, will it look radically different from a user who accesses it from an iPhone 6? Do I have to keep this in mind as I design the interface? I’m sure this is a common question but wasn’t able to find a quick answer to it. Any information would be greatly appreciated!

Short answer yes it will look different, but you have total control on what happens through responsive.

Watch the responsive video it will explain.