Explanation how DataTable ColumnGroup works

[dcarapic]

For the life of me I can not understand how DataTable ColumnGroup works (https://www.primefaces.org/primevue/datatable/colgroup).
Whatever I do the columns I define are not grouped and are often duplicated.
Do I have to repeat each column within the ColumnGroup/Row or not?
Where do I specify the column caption and where do I specify the data?