After you first create the droplet with dokku installed and you go to your droplet ip address in the browser, you should see the Dokku Setup screen. You have 2 options here:

1. Hit the checkbox to use virtualhost naming for apps. This allows dokku to make your apps available as subdomains instead of as port numbers of your droplet IP

2. Leave the checkbox unchecked to access your apps as different port numbers of your ip

For option 1, you will need to purchase a domain name and add the digital ocean nameservers and configure your DNS records. Then you can access your apps at the subdomains (ex. app1.dokku.com, app2.dokku.com). I’m not sure how option 2 works exactly. Not sure how the port numbers are managed for your different apps.

This tutorial is where I found this info: https://maximilianschmitt.me/posts/tutorial-deploy-apps-websites-dokku/#3-completing-the-dokku-installation

Hope this helps