This is the most reliable "official" way. You download the game's .ipa file from a trusted source and use a tool like AltStore or Scarlet to install it.
While does not have an official version on the Apple App Store due to strict publishing restrictions, it is possible to play the game on iOS devices using verified community-vetted methods. Because the official Summertime Saga Website only provides builds for Windows, Linux, macOS, and Android , iPhone users must rely on alternative distribution. Verified Methods for iOS Installation
When you search for "verified" downloads on Google or YouTube, you will typically encounter three methods. Here is a review of their legitimacy: