Unity `AddFileToBuild` How to achieve？

Unity `AddFileToBuild` How to achieve？
```csharp
// https://docs.unity3d.com/ScriptReference/iOS.Xcode.PBXProject.html
proj.AddFileToBuild(target, proj.AddFile(fileName, fileName, PBXSourceTree.Source));
```

The documentation is too smal.

Start by reviewing the PBXProject AddFileToBuild example referenced in the issue and the current project documentation. Clarify the intended Unity/iOS usage, then document a complete working example and verify that the explanation answers how to add the file to the build.

Written by the indexing model from the issue text.