You are copying just a field in an item. This will not copy other items, like related datasources.
However, the relationship to the datasource items will remain as this is based on guids inside the final renderings field.
I think based on your code sample we can assume you are copying final renderings between language versions. In that case, the datasource relation will be fine. It will still point towards the original datasource but Sitecore will take the context language when rendering it.
Along with the answer given by @Gatogordo, want to add this like If you want the list of data sources used in your rendering, then you can use the following Powershell Command, which will give you the output of the data source used.