Go to Settings > General > iPhone Storage to see what’s eating your storage.
You might find some real surprises here (for example, innocuous things such as stored iMessage messages can eat up a crazy amount of space). Other times, you’ll find an app that you don’t use that much that consumes a huge chunk of space.
Here you have two options. The nuclear option of deleting the app and the data completely, or the more restrained option called Offload App, which allows you to delete the app but keep the data, so when you later download it, you have all the untouched.