Text retrieved from TextFormField is not showing in Flutter

You have to pass the data from the initial screen to the new screen, so that you can render the data.

Here's flutter resource on how to pass data between 2 screens.

https://docs.flutter.dev/cookbook/navigation/navigate-with-arguments

From the documentation,

You can accomplish this task using the arguments parameter of the Navigator.pushNamed() method. Extract the arguments using the ModalRoute.of() method or inside an onGenerateRoute() function provided to the MaterialApp or CupertinoApp constructor.

Below is a simple prototype that should do the trick. Check the screenshots and the DartPad prototype:

1st page	2nd page

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      initialRoute: '/home',
      routes: {
        '/home': (context) => const HomePage(),
        '/next': (context) => NextPage(
              ModalRoute.of(context)!.settings.arguments as String,
            ),
      },
    );
  }
}

class HomePage extends StatefulWidget {
  const HomePage({Key? key}) : super(key: key);

  @override
  State<HomePage> createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  final customerNumber = TextEditingController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Pass Args Between Routes Demo'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Padding(
              padding: const EdgeInsets.all(20),
              child: TextFormField(
                style: Theme.of(context).textTheme.headline4,
                autofocus: false,
                keyboardType: TextInputType.number,
                controller: customerNumber,
                textInputAction: TextInputAction.next,
                decoration: InputDecoration(
                  prefixIcon: const Icon(Icons.phone),
                  contentPadding: const EdgeInsets.fromLTRB(20, 15, 20, 15),
                  hintText: "Enter Customer Number",
                  border: OutlineInputBorder(
                    borderRadius: BorderRadius.circular(10),
                  ),
                ),
              ),
            ),
            ElevatedButton(
                onPressed: () => Navigator.pushNamed(context, '/next',
                    arguments: customerNumber.text),
                child: const Text(
                  'Next',
                  style: TextStyle(fontSize: 30),
                ))
          ],
        ),
      ),
    );
  }
}

class NextPage extends StatefulWidget {
  final String customerNumber;
  const NextPage(this.customerNumber, {Key? key}) : super(key: key);

  @override
  State<NextPage> createState() => _NextPageState();
}

class _NextPageState extends State<NextPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Pass Args Between Routes Demo'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Padding(
              padding: const EdgeInsets.all(20),
              child: Text(
                widget.customerNumber,
                style: const TextStyle(fontSize: 30),
              ),
            ),
          ],
        ),
      ),
    );
  }
}