How to position a widget outside ModalBottomSheet in flutter

What you can do is create the bottom sheet as a column of two containers, and the first (bottom) container with transparent background. This way you can overcome the problem because the transparent container will be a part of the bottom sheet, and since you can put the icon (circle avatar etc.) "above" this container with Stack, the icon will be visible.

So combine Stack and Positioned to achieve the result:

import 'package:flutter/material.dart';

void main() => runApp(const BottomSheetApp());

class BottomSheetApp extends StatelessWidget {
  const BottomSheetApp({super.key});

  @override
  Widget build(BuildContext context) => MaterialApp(
        home: Scaffold(
          appBar: AppBar(title: const Text('Bottom Sheet Sample')),
          body: const BottomSheetExample(),
        ),
      );
}

class BottomSheetExample extends StatelessWidget {
  const BottomSheetExample({super.key});

  @override
  Widget build(BuildContext context) => Center(
        child: ElevatedButton(
          child: const Text('Show ModalBottomSheet'),
          onPressed: () {
            showModalBottomSheet<void>(
              context: context,
              backgroundColor: Colors.transparent,
              builder: (BuildContext context) {
                return SizedBox(
                    height: 264,
                    child: Stack(children: [
                      Column(children: [
                        Container(
                          height: 64,
                          color: Colors.transparent,
                        ),
                        Container(
                            height: 200,
                            width: double.infinity,
                            color: Colors.white,
                            child: Column(
                                mainAxisAlignment: MainAxisAlignment.center,
                                children: [
                                  const Center(child: Text('Well done')),
                                  const SizedBox(height: 8.0),
                                  ElevatedButton(
                                    child: const Text('Close BottomSheet'),
                                    onPressed: () => Navigator.pop(context),
                                  ),
                                ]))
                      ]),
                      Positioned(
                          left: MediaQuery.of(context).size.width / 2 - 32,
                          top: 32,
                          child: const CircleAvatar(
                            radius: 32,
                            backgroundColor: Colors.green,
                            child: Icon(Icons.check, size: 24),
                          )),
                    ]));
              },
            );
          },
        ),
      );
}