Thank You very very much fot the answer that is not what I have seen on other places, and not in the utser manual.
What have you seen on other places? I dare say you've never seen a web server that takes content placed at one URL, and makes it available at other random URLs, without being explicitly told to--and that's what you're asking for here.
You shouldn't expect anything about phpmyadmin to be in the user manual, because it isn't part of the base SME Server distribution. It's a contrib, packaged by a third party for use with SME Server, and documented on its own wiki page (
https://wiki.contribs.org/PHPMyAdmin). I guess that page could mention that it's available at
https://yourserver/phpmyadmin (which the wiki already says), and not at
https://yourserver/$RANDOMIBAY/phpmyadmin (which it doesn't). It could also mention that the contrib is not known to have caused the death of any small kittens, which would make about as much sense. There's simply no reason to expect that a web app that's located at one URL will be accessible at any other URL.
If you have more than one domain set up for your your SME server, phpmyadmin is accessible at all domains--https://www.firstdomain.tld/phpmyadmin,
https://www.seconddomain.tld/phpmyadmin, etc. But it isn't going to be accessible under any subdirectory unless you put it there. Or, if you want different users to have access to different databases, enable multiuser access for phpmyadmin (it's described on the wiki page), and have the users log in with a user/password suitable for the database(s) you want them to have access to. But you haven't said what you're trying to accomplish, I'm not sure if either of these will help you.