private extension VerticalAlignment {
  private enum Anchor : AlignmentID {
    static func defaultValue(in d: ViewDimensions) -> CGFloat {
      return d[VerticalAlignment.center]
    }
  }
  static let anchor = VerticalAlignment(Anchor.self)
}

#Preview {

  ZStack(alignment: .init(horizontal: .center, vertical: .anchor)) {
    Color.clear
    VStack(alignment: .leading) {

      Rectangle()
        .frame(width: 200, height: 200, alignment: .center)
        .alignmentGuide(VerticalAlignment.anchor, computeValue: { d in d[VerticalAlignment.center] })

      Rectangle()
        .frame(width: 200, height: 200, alignment: .center)

    }
  }
  .background(Color.yellow)

}

<aside> 💡 Color.clear is super important to expand ZStack as full size. even frame modifier as infinite size does not work. expanding element will be the key.

</aside>

SwiftUI: Pin to the top & bottom of a centered element