Building Plugins for iOS is a guide from Unity.

Two approaches

• Approach 1: Put plugin source code .mm,.c,.cpp files in Assets/Plugins/iOS folder.
• Approach 2: Build plugin library to .a file then put it in Assets/Plugins/iOS

How to build Unity3d Plugin for iOS experience from a developer

Plugin for Unity and iOS iOS plugin with source code inside Asset folder

Unity3D Plugins Development Guide