In the above, Plone is the ID of the site, and it is part of the URL.
When you add more sites to that installation, you give them different IDs, so if you created Plone2, that new site would be at http://araadhnatravels.herokuapp.com/Plone2 and so on.
The Heroku button is really meant for testing/trying out only. Normally, you would not expose the management interface to the public. Instead, you would put a web server like nginx in front of your Plone installation, and you would configure nginx so that browsing to https://mydomain.com causes nginx to rewrite that to http://localhost:8080/Plone
You can do this in the VHM mappings tab in the ZMI but it will require you to have another domain name, otherwise you might lose access to the root of the zope instance which would be bad.