I have just downloaded wordpress from wordpress.org and installed it on my hosting. Then when my site has already saved and published, when I open it from another phone and computer I get the pop-up advertisement in top-right corner. I created the site without any free themes that might have ads. I use wordpress 5.2.3. ...So if someone had that problem too, please response. I need the site immediately. I have never seen this problem before and on whole internet no answers about it.
That does not sound like a problem created by wordpress. You could try renaming index.php to index.bak and create an empty index.php. If you open the new index.php an the ad still appears, at least you will know the problem does not lie within wordpress‘ code.