A client of our recently received a notice from Google Search Console (GSC) about "Search Console has identified that your site is affected by 2 new issues of type Mobile Usability" and asked us to take a look.
It turns out the error was coming from an attachment page (the page for the image file), which led us into the following explanation about what content should be indexed and what shouldn't
So in WordPress, there's pages, posts and media. As you'd expect, each page and post has its own URL. So far so good.But for media files, WP sees each media file (image, etc) as it's own element, so in WP you really have a list of pages, posts, images, etc. And the reason they do it this way is because your images can get indexed separately from your website (as when someone does a google image search). This is also one of the reasons that if you were to look at your sitemap file, you'd see multiple sitemap (which is the intended behavior)