You need to publish your app on your own account (or your customer's account*) in the Google Play Store and the Apple App Store.
Publishing apps to the Apple App Store requires an Apple developer license. This costs $99 per year.
A Google Play Developer license is $25 (one-time fee).
* If you are building an app on behalf of a company, it is generally a good idea to have that company provide their own Google Play and iOS Developer licenses.