About your question here on setting up a grouped calendar, you will first need to create the individual calendars you would like to include. Once those calendars have been created, you can then add them to a Grouped Calendar.
Regarding your query here, actually you do not need to copy and paste anything. If you have already created and published your other calendars, then clicking on that field, will instantly avail the calendars for selection. Here is a sample screencast: https://drive.google.com/file/d/1kd-VjR7if-UBWW01SrGVxzTdJPKx8yKp/view?usp=drivesdk . Please ensure that you have created and published your other calendars first, as guided in the docs I shared.