With the manufacturing of CDs, the label pays a fee to the PRO (or direct to the publisher, USA / Latin America only) for every CD pressed. The PRO distributes this to all songwriters on that release.
Digital shops pay 10% over the gross sale per release to the PRO (or direct to the publisher, USA / Latin America only). That amount is split and distributed to all contributing songwriters.
Example: When iTunes sells an album for $11.99, $1.99 goes directly to the PRO (or publisher, USA / Latin America). That $1.99 is then split among the songwriters on the album.
Cash flow