There are 2 credit memo grids in the backend:
In the «Sales» → «Credit Memos» section:
On the «Credit Memos» tab of an order screen:
Magento\Sales\Ui\Component\Listing\Column\Price::prepareDataSource():
Data source: sales_order_creditmemo_grid_data_source
It is implemented by the Magento\Sales\Model\ResourceModel\Order\Creditmemo\Grid\Collection
class: