SD card is a thing that we use to increase the storage in a phone. Because many times we may need to increase the storage of our phones even though the phones come with very large memory. Not all phones have a card slot for using SD cards.
Oppo Reno8 pro (china) phone comes with 128GB and 256GB phone storage which is enough to keep our files and various documents, and I think there will be no need to expand storage in this phone. However, if you want to increase its storage, you cannot increase its storage as the phone does not provide any SD card slot.