Yes, lots of themes and plugins make use of the wp-admin/admin-ajax.php file, and some use wp-admin/admin-post.php. But beyond that there’s little to no reason to access wp-admin.
Perhaps a search engine trying to determine if I’m running wordpress?
Search engines don’t care so much if you are or aren’t using WordPress, you would be able to tell via the user agent if it was a searchbot. If it is, then this implies you have links to nonexistent files in wp-admin that would show up in tools such as google web console, etc.
But if they did care, which they don’t, there are far better ways to identify a WordPress than querying wp-admin.
What this is likely to be, are automated attacks. Almost all hacking attempts are automated, they don’t even check your site. Exploits are fire and forget. Why make requests to a site to figure out if the exploit will work when you can just run them all? If one of them works that’s all it takes.
For this reason, you’ll see a lot of login attempts and exploit requests that aren’t even for your software. My WordPress sites get Drupal exploits and attempts to login to Joomla URLs, and I ignore them. Iff they ever became an issue I would block them at the Nginx level or lower.
At the end of the day, we cannot know for certain. If you are concerned you might break things, contact the community around your CMS and ask them. For all I know you use a CMS called WonderPage that just happens to have a wp-admin folder