When I use a code like this to present a ShareSheet

import SwiftUI

extension UIApplication {
  static let keyWindow = keyWindowScene?.windows.filter(\.isKeyWindow).first
  static let keyWindowScene = shared.connectedScenes.first { $0.activationState == .foregroundActive } as? UIWindowScene  
}

extension View {

  func shareSheet(isPresented: Binding<Bool>, items: [Any]) -> some View {
    guard isPresented.wrappedValue else { return self }
    let activityViewController = UIActivityViewController(activityItems: items, applicationActivities: nil)
    let presentedViewController = UIApplication.keyWindow?.rootViewController?.presentedViewController ?? UIApplication.keyWindow?.rootViewController
    activityViewController.completionWithItemsHandler = { _, _, _, _ in isPresented.wrappedValue = false }
    presentedViewController?.present(activityViewController, animated: true)
    return self
  }

}

struct ContentView: View {

  @State private var isPresentingShareSheet = false

  var body: some View {
    Button("Present Share Sheet") { isPresentingShareSheet = true }
      .shareSheet(isPresented: $isPresentingShareSheet, items: ["Share me!"])
  }

}

A button named COPY is always present on the sharesheet itself.

When I press copy, what exactly is being copied and from where?

If this copy means the text or element that is selected when the sharesheet is triggered this creates me a problem, because I want to pass a rich text that is composed of a title, an image and a long description.

Can I define a function to run when that button is pressed?

Hello,

yes, you can customize this, but not by running a function. Instead you need to create custom "wrapper" around the data you pass to the share sheet that conforms to UIActivityItemSource .

You can then do something like this:

func activityViewController(_ activityViewController: UIActivityViewController, itemForActivityType activityType: UIActivity.ActivityType?) -> Any? {
    if activityType == .copyToPasteboard {
        return richText
    } else {
        return standardText
    }
}

Feel free to check more tips about share sheet on my blog - https://nemecek.be/blog/189/wip-sharing-data-with-uiactivityviewcontroller-tips-tricks