Align widget
To align a child widget within its parent you use the Align widget. If you've used the Center widget, you know it's a special case of Align.
Alignment class in the Align widget usually aligns the widget with it taking reference to the center of the parent widget.

~ Alignment(-1.0, -1.0) represents the top left of the rectangle and is same as Alignment.topLeft.
~ Alignment(1.0, 1.0) represents the bottom right of the rectangle and is same as Alignment.bottomRight.
~ Alignment(0.0, 3.0) represents a point that is horizontally centered with respect to the rectangle and vertically below the bottom of the rectangle by the height of the rectangle.
~ Alignment(0.0, -0.5) represents a point that is horizontally centered with respect to the rectangle and vertically half way between the top edge and the center.

~ Alignment(x, y) in a rectangle with height h and width w describes the point (x * w/2 + w/2, y * h/2 + h/2) in the coordinate system of the rectangle.
import 'package:flutter/material.dart';

void main() {
  runApp(
      const MaterialApp(
          debugShowCheckedModeBanner: false, 
          home: MyApp())
      );
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});
  @override
  Widget build(BuildContext context) {
    return SafeArea(
      child: Scaffold(
          body: Container(
        width: MediaQuery.of(context).size.width,
        height: MediaQuery.of(context).size.height * 0.5,
        color: Colors.blue[100],
        child: Align(
          alignment: Alignment.topRight,
          child: Text(
            "Hello".toUpperCase(),
            style: const TextStyle(
                fontSize: 30.0, 
                fontWeight: FontWeight.bold
                ),
          ),
        ),
      )),
    );
  }
}
Now, let's say we want the Text widget aligned to the right at the edge and at the bottom just outside the Container widget, now we pass a pair of double values to the Alignment constructor, Alignment(1.0, 1.2).
child: Align(
  alignment: const Alignment(1.0, 1.2), // left, top
  child: Text(
    "Hello".toUpperCase(),
    style: const TextStyle(
        fontSize: 30.0, 
        fontWeight: FontWeight.bold
        ), // TextStyle
  ), // Text
), // Align
So far, we've been using center-oriented reference for aligning child widgets in the rectangle. If however, we'd want to use a coordinate system with an origin in the top-left corner of the Container, we can use the FractionalOffset class.

In Alignment class, 0.5 span accounts for a quarter of a Container, in FractionalOffset, 0.5 span accounts for half of a Container.

The codebase below centers the child widget in the Container using the origin as the top left.
child: Align(
  alignment: const FractionalOffset(0.5, 0.5), // left, top
  child: Text(
    "Hello".toUpperCase(),
    style: const TextStyle(
        fontSize: 30.0, 
        fontWeight: FontWeight.bold
        ), // TextStyle
  ), // Text
), // Align