I need to test something on the old phone and unity throws error that device is not supported “The connected device is running android with API level 15, while minimum API level of the project is set to 22.” the problem is that 22 is minimum API level that I can even set there are no lower versions. My question is how to install old API version in this case API level 15?
I did something similar long ago, and I remember downloading and installing the SDK platform and tools from the Android SDK Manager. Afterward, I went to the “Player Settings” in Unity and changed the Minimum API Level. I hope this is relevant.
Download android studio and find the apk you want. It will automatically install for you.
Edit: API 15 isn’t supported on the current/latest version of the Unity editor. If you really need to, download an older editor
See: Why i can't select minimum android api level 15 from minimum api level dropdown (unity 2017 or higher) - Stack Overflow