This is a simple UI that demystifies Stack

simple-ui.dart

import 'package:flutter/material.dart';

void main() => runApp(const MaterialApp(home: MyApp()));

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);
  
  @override
  Widget build(BuildContext context) {
    return Scaffold(
    body: Center(
    child: Stack(
    alignment: Alignment.topCenter,
    clipBehavior: Clip.none,
    children: [
      Container(
      padding: const EdgeInsets.fromLTRB(25, 70, 25, 30),
      color: Colors.green.shade50,
      child: const Text(
      'Successful',
      style: TextStyle(
      fontSize: 27,
      color: Colors.green,
      ),
      ),
      ),
      Positioned(
        top: -20,
        child: Container(
        padding: const EdgeInsets.all(16),
        decoration: BoxDecoration(
        color: Colors.white,
        border: Border.all(
        color: Colors.green.shade50,
        width: 4,
        ),
        shape: BoxShape.circle,
        ),
        child: const Icon(
        Icons.gpp_good,
        size: 40,
        color: Colors.green,
        ),
        ),
      ),
    ],
    ),
    ),
    );
  }
}