There is no downloadable application for smartphones. No adult sites have it as they are banned from the Google Play and Apple stores as adult content cannot be part of these apps.
This does not mean that sites like OnlyFans are not mobile friendly. In fact, they are designed to be viewed primarily on mobile devices. Many people set up a tagged link and add it to their phone's home screen, to work almost like an app.
Therefore, you will not find any official apps in the app stores for your phone. Anything you find will be unofficial and should be avoided.