problems with QGraphics in qt c++ widgets

Question

I just started learning qt widgets and C++ in general, so I don't understand a lot. I created a regular project, made a full-screen view, created a scene inside it, and set a square inside the scene. I wanted to set view, scene, rect to different colors, but for some reason I can't do it. I either have the entire view painted in the color of the scene or vice versa

1)main.cpp

#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    MainWindow mainWindow;
    mainWindow.show();
    return app.exec();
}

2)mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QGraphicsItem>

class MainWindow : public QMainWindow
{
    Q_OBJECT

    public:
        MainWindow(QWidget *parent = nullptr);
        ~MainWindow();
};
#endif // MAINWINDOW_H

3)mainwindow.cpp

#include "mainwindow.h"
#include <QGraphicsRectItem>

MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent) {
    // Create QGraphicsScene
    QGraphicsScene *scene = new QGraphicsScene(this);

    // Set the scene size (in this case, for example, 300x300)
    scene->setSceneRect(0, 0, 300, 300);

    // Installing a brush (color lantern) for scenes
    scene->setBackgroundBrush(QBrush(Qt::green));

    // We create a QGraphicsRectItem (a rectangle with the boundaries of the scene) and deliver it to the case
    QGraphicsRectItem *sceneBounds = new QGraphicsRectItem(scene->sceneRect());
    scene->addItem(sceneBounds);

    // Create a QGraphicsRectItem (rectangle) and add it to the scene
    QGraphicsRectItem *rectangleItem = new QGraphicsRectItem(50, 50, 100, 100);
    scene->addItem(rectangleItem);

    // Set the brush (the color is filled) for the outline
    rectangleItem->setBrush(QBrush(Qt::yellow));

    // Create a QGraphicsView and set the danger to it
    QGraphicsView *view = new QGraphicsView(scene);

    // Set the background color for QGraphicsView (can be any color that does not affect the case)
    view->setBackgroundBrush(QBrush(Qt::lightGray));

    // Set the scene alignment in the upper left corner
    view->setAlignment(Qt::AlignLeft | Qt::AlignTop);

    // Set QGraphicsView as the central widget of the main window
    setCentralWidget(view);
}

MainWindow::~MainWindow() {}