There is no Spotify Premium APK for iOS Devices. While it is true that in Android we can install APK files from diverse sources, on iOS the process of installing applications additional to those offered by Apple has a more giant limitation. Applications on iOS must be downloaded and installed via the official Apple App Store, which does not have an APK-based application upload format. Therefore iOS users can not directly install spotify premium apk!
The App Store only has Spotify app, that is the only valid way to get a premium account of Spotify on iOS. This is same as the Premium APK but it needs to comply with Apple app deployment limits. Spotify does add all the Premium features — offline listening, ad-free streaming etc. — to its official iOS app though.
Furthermore, using Spotify Premium on iOS with third-party apps that major in unofficial methods to get around Apple's restrictions raise some serious security concerns. For instance, some 5% of iOS users looking for unapproved ways may fall victim to malware or data leaks. Apple addresses this threat by only enabling app installations from verified sources through their thorough security process.
In order to get Spotify Premium on iOS, users need to subscribe through the official Spotify app in App Store. This app ensures that users get the latest features and updates from Spotify itself like Streaming in quality or security enhancements.
While you are at it, do well to checkout Spotigeek for more on official Spotify premium features and updates. In this resource, you will find in-depth knowledge on using Spotify Premium to the fullest as well as always staying up-to-date with all of its features.