Seems a custom template is assigned ( right hand side bottom Page Attributes >> Template ) to Movie Grid page and your page is displaying from that template as from backend it’s blank.
So if you wish to create replica of this page, you need to assign this template to your newly created page. But you might need same structure but different data / listing on your new page.
For that, you need to create new custom template, change the code in that, may be if wp_query is used, you may need to just change the parameters depending upon what data you need to display. Assign that custom template to newly created page. If you are using any paid or contributed theme, make sure you edit / create your new code in your child theme.
1. when i add any other element like simple messagebox or anything else it doesn’t work
>> Check your custom template (whichever assigned) once. If that file contains the_content() function written inside while loop, then whatever you add from backend, it should display on frontend.
If not, this means your content is not being called in your custom template. You can add the_content() inside while loop where you want to show the content that has been added from backend.
Check for loop
Hope this helps.