I want to create a background with an outer frame.

How to expand to the full screen

Change AppCompatActivity to Activity and Remove Title Bar

Change the theme

@android:style/Theme.NoTitleBar

@android:style/Theme.NoTitleBar.Fullscreen

https://akira-watson.com/android/theme-notitlebar.html

I don't think the title matches the question.
"I will reply saying ""The application display area does not expand to the entire screen of the device.""""

First, specify a theme that does not have a title bar in XML like the default a.j.w answer, or
Or call the Activity class requestWindowFeature (Window.FEATURE_NO_TITLE) over the java code.

Next, clear the status bar and navigation bar.

View decor=this.getWindow().getDecorView();
clear decor.setSystemUiVisibility(View.SYSTEM_UI_FLAG_HIDE_NAVIGATION//Navigation area)
                           | Clear View.SYSTEM_UI_FLAG_FULLSCREEN//Statusbar Area
                           | View.SYSTEM_UI_FLAG_IMMERSIVE_STICKY); // Set the immediate mode

NaviBar, StatusBar cannot be permanently deleted and will revert with Swipe at the bottom of the screen (Android specification)

If your question is not
The application display area does not expand to the entire screen of the device,
Screenshot of the current problem screen and images of expected behavior,
and layout.xml, etc. I think I can give you an accurate answer.