One copy of Adminer installed on a single app on your server will allow you to access all of your apps' databases; however, if you don't want Adminer installed on one of your existing apps, you can install it on its own with the same result.
Installing Adminer on its own app has some benefits, including access through a subdomain rather than from a subdirectory and the elimination of potential conflicting .htaccess rules installing it on an existing app could cause.
First, create a new app for Adminer in ServerPilot.
Enter a subdomain for the app, such as db.example.com (or whatever you want it to be). Don't forget to configure DNS for this subdomain.
To install Adminer, visit www.adminer.org and click Download.
Then, click the first selection in the Downloads list; this should be the most recent version of Adminer.
Adminer will begin downloading from SourceForge.net.
When the download completes, drag adminer-X.Y.Z.php (where X.Y.Z is the version number) from your computer's Downloads folder to your app's public folder in Cyberduck or your preferred SFTP client.
Delete your app's original index file and rename adminer-X.Y.Z.php to index.php.
You should now be able to bring up Adminer through this subdomain (e.g., http://db.example.com) in your browser.
Again, you can easily access all of the databases on your server by signing in with their correct credentials through this singular Adminer installation and subdomain.
You can log in to Adminer using any database name, username, and password that you've created through ServerPilot for any app on this server.