Is that a malicious code in SQL

Yes, Alphart, this is some adware that redirects visitors to adverts.

The malicious code is being appended to all your posts, all your pages and also to your individual CSS (Customizer, Additional CSS). To make the code visible you have to switch the block editor to HTML editing.

The adware redirects only mobile devices (I guess because it is more difficult to check HTML there and find the code).

After you deleted the malicious code from all documents it will still be in your database as long as you keep older versions of your document. You can erase all those old versions using the plug-in “WP-Sweep”.