TL;DR:

1. The application creates a wl_data_source object, indicating that it’s going to offer data to other applications.
2. The application adds the mime types that it can handle to the data source (with wl_data_source::offer)
3. The application finally calls wl_data_device::set_selection, to indicate “I’m taking ownership of the clipboard, and the above created wl_data_source is what I’m offering”.