URLs that don't exist should display an error page and return a 404 (not found) response.
When you have made-up URLs going to an actual page that returns a 200 (OK) response, the problem actually is that Google will start seeing this as soft 404 errors, which is not good.
From Google Search Console: "Returning a code other than 404 or 410 for a non-existent page (or redirecting users to another page, such as the homepage, instead of returning a 404) can be problematic. Firstly, it tells search engines that there’s a real page at that URL. As a result, that URL may be crawled and its content indexed. Because of the time Googlebot spends on non-existent pages, your unique URLs may not be discovered as quickly or visited as frequently and your site’s crawl coverage may be impacted..."