Hi @FriendsMissRiv I agree with @BrendanNewell, if you can update the device that would be a good first step, but some older devices may not support iOS 15 (I know I have an older iPad that can use anything more recent than iOS 12). The iPhone 6S, however, is compatible with iOS 15, so the device you are testing on should be upgradable. In general, most sources report that a very large percentage of iPhone owners are on either the latest major OS or the version before that, so this issue shouldn't be that widespread.
In terms of making your story more mobile friendly, ArcGIS StoryMaps does a lot of that for you. If you upload your images, the builder automatically optimizes them for the web and serves out specifically sized versions for mobile devices vs. desktop readers. Also be sure to add alternate images for any embeds so they look good on small screened devices (the builder prompts you to do this). Immersives are designed to work well on smaller screens. The experience will be a bit different, but the content layout is adjusted to try to give readers on small screens the best experience possible give the space constraints. You should always test out what your story will look like on phones and tablets using the multi-device preview mode that's available in the builder.
Owen Evans
Lead Product Engineer | StoryMaps