so when I type http://localhost:5000/ in the “add podcast by url” field, the app adds a bit (shown in the attached screenshot) that should not be there which makes it fail to grab the feed.
Did you install podcats on your phone or on a computer on the network? If it is another device, localhost will not work. You need the hostname or IP of the computer that runs the service.
The ::1 part is the IP address of your local device (127.0.0.1). AntennaPod did not edit the url, it just translated the word localhost into an IP.