A user story is defined in quite a few ways, but roughly always like this:
User Story is a small (actually, the smallest) piece of work that represents some value to an end user and can be delivered during a sprint.
For some features, this is obvious. It could be something like a new filter or the ability to sort a product list. But what when it's less obvious - particularly what if you are building something from scratch?
For a new application (no matter if it's a mobile app, web app, or anything else) the smallest piece of work that represents some value to the end user, is still quite a large piece of work. You need to setup a UI (even if only basic), create a new database, add the most basic functionality, etc. Way larger than what I would consider a user story.
So how do you use user stories in the very first stages of developing something new?