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Do I need JPanel always?

I am now writing code simple GUI that's for start the game window. I only need Do you want to start game message and start button on the window. But I have a confusing concepts for the JFrame and JPanel. Actually, I thought I need to add JPanel to JFrame to add the other components such as JLabel, JButton,...etc. But I realized I don't actually need JPanel. I can just add the components simply use add(button), add(label) to JFrame. So why I need JPanel. And I think JFrame doesn't need JPanel but JPanel need JFrame. Am I understand correctly?

No, not always. A simple graphical user interface may be implemented by just adding components "directly" to a JFrame. But in order to get more flexibility, you would always use JPanels. For example, to employ different layouts in different parts of the GUI, to group certain components together, etc.
A JFrame is backed by a JRootPane, a part of which is a contentPane.
(image from Oracle Javadoc)
When you add components to a JFrame, you are really adding them to the content pane, e.g.: frame.getContentPane().add(Component).
A JFrame is a common starting scene of a Swing GUI application, while a JPanel is intended to be put in another scene (container). Since both content pane and a JPanel inherit from the same class (Container) you may use them in a similar manner, as far as adding components to them goes.

Do I need JPanel always?
No. Well, unless you need a Swing GUI. Then yes.
Another answer replied words to the effect. "No, you can add components direct to a frame" What they missed was that components added to a JFrame are added to the content pane (automatically). The content pane is a JPanel.
Having said that:
I (and many others) would recommend designing an app based around a main content panel, then adding that panel to a top-level container as needed. The top level container might be a JFrame, JWindow, JDialog, JOptionPane ..
What prompted the question? A JPanel is a very 'light weight' container (in more ways than one). A GUI can contain 1000s and not be burdened by doing so. Of course, that's a rare requirement, but just saying .. use panels as needed and don't worry about it.

How can I create responsive JList in Java Swing

I want to create a shop by having a main JPanel that each component inside it is a JPanel with an image, label and button.
I did tried using a JList but the problem with the JList its only holds the rendering of the component and because of that the button isn't working and its only an image. I can walk around and use MouseEvent but it feels wrong for me and I am sure that there is a better solution for it.
I want that the components will change their positions depend on the frame size, like in the JList.
For example, if I change from the width of the screen the positions of the components will change from this:
to this:
I do have an idea by using GridLayout or GridBagLayout in the paintComponent (because it calls every rendering. If you know another method that calls every rendering int the JPanel I would like to know) and changing the positions of the components by changing the layout variables inside the paintComponent.
I did surfed the internet to find a solution but I only found that people used JTable but I don't see it working here.

each component inside it is a JPanel with an image, label and button.
Makes sense.
changing the positions of the components by changing the layout variables inside the paintComponent.
The paintComponent() method has nothing to do with changing the layout of the panels. You should not be playing with the paintComponent() method.
I do have an idea by using GridLayout or GridBagLayout
You are correct to use a layout manager, but unfortunately, none of the default layout managers will wrap automatically at a random number of components.
The layout managers are invoked automatically as the frame is resized.
So you can use the Wrap Layout which is an extension to the FlowLayout that will allow random wrapping.

Swing: how to paint an animation over every component, JPanel, JButton, etc?

I'm trying to paint over every component in my Swing application.
what I have:
jButton b = new JButton();
b.addActionListener(e -> fillEntireScreen())
f.add(b);
f.setVisible()...f.setSize()...f.setDefaultCloseOperation()...
where fillEntireScreen() simply is an animation that expands over the screen. I override Component's paintComponent(), painting over the frame, not a panel or anything.
fileEntireScreen() works without the button, but does not work when I add the button, as the animation is behind the button.
how can I paint over the button?
Thanks!

Use a glassPane instead - See How to Use Root Panes for more details and How can I paint in an specific JPanel when more than one in same frame- Java for an example.
Alternatively you could use a JLayer, but's more complicated and may not suit your needs. See How to Decorate Components with the JLayer Class

Painting a JComponent without adding it to a container

I've implemented a custom JPanel, whose paint method I've extended to do a lot of manual rendering in full screen mode. Now I would like to integrate another JComponent to this (in my case a JPanel that contains a JScrollpane with a JTextPane as its viewport) that should appear on top of my first panel, but because my custom rendering pipeline is complex, adding the JComponent to my panel and having it painted the traditional way through the AWT system is not an option (I tried and it's quirky at best, not functional at worst), so my question is: is it possible to manually order the JComponent to be painted at one point in my program by calling its regular paint method without tying it to a JContainer and if yes, how do I do this?
Thanks in advance for your answers.

See the LabelRenderTest.java source on this thread. The label is eventually drawn to screen, but it is painted to BufferedImage before ever being displayed.
The important line of the source is..
textLabel.setSize(textLabel.getPreferredSize());

You can take a look at CellRendererPane and see how for example BasicTableUI paints component images with it.

Yes, just call the normal paint method on the object and pass the Graphics you want it to paint on. However, this is just going to paint it and it sounds like you want it to possibly scroll which means you will need to add it to your custom JPanel. In that case just add the panel and you a layout manager that will place the component where you need it.

You should set size for the component. Then to position it use your Graphics' translate(x,y) to position the component in desired Point.

if there is any container higher level in the hierarchy you can use
validate(); repaint();
pair to do that.
if not you can change it's size or bounds ( like +1 , -1 ) at the end to make it repaint itself.

Java - control Z order of JPanels

In short, my need is to have a background Image in my java app, and upon some event, create some other graphics on top of that image.
I was thinking I would use a JPanel to draw the background image in, add it at to my JFrame the start of program, and then add other JPanels on top of that upon certain events. The problem is Swing gives the JPanels added first the highest Z index, so what should be my background is showing up on top of everything.
Is there any way to control the Z index/order of the JPanels, or am I going about this completely wrong?

You can use the setComponentZOrder() to handle Z-Order in your application.
Resources :
JavaDoc - Container.setComponentZOrder
oracle.com - Mixing heavy and light components

Sounds strange to add mutiple JPanels and use z-order. I would suggest you either simple add ONE JPanel with the paintComponent(Graphics g) method overwritten, where you draw the correct image according to your events.
Or use the CardLayout, and add JLabels (with different images), then when your event triggers, use
CardLayout cl = (CardLayout)getLayout();
cl.show(this, "card3");
to show the correct JLabel.

The JLayeredPane is designed for just this purpose. It allows you to control the Z-order of components.

I was thinking I would use a JPanel to
draw the background image in, add it
at to my JFrame the start of program,
Sounds reasonable.
and then add other JPanels on top of
that upon certain events. The problem
is Swing gives the JPanels added first
the highest Z index, so what should be
my background is showing up on top of
everything.
Adding a panel to a panel is just like adding another component to the panel. The child component will be painted on top of the parent panel. There is no need to play around with Z-Order.
The key is to set a layout manager on the panel with the image. Then each panel you add to the image panel must be made non-opaque so that the background image will be painted.
Maybe the Background Panel can help you out. It automatically makes any component added directly to the panel non-opaque.