The MKR MEM shield allows a MKR board to read and write an SD card using the SD library and also contains 2MB of serial flash memory available for your program data or OTA updates. Flash and SD are managed on the same SPI bus and should be selected through the proper CS pin.
Refer to the SD library for usage examples of the SD memory card. Refer to the Paul Stoffregen's SerialFlash lib to find how to use the onboard 2MB serial flash memory. Please note that on the MKR MEM Board, the Serial Flash uses CS pin 5 instead of 6. All the library examples from Paul's lib need this CS pin changed from 6 to 5.
You can download it using the Library Manager available in the Sketch -> Include library dropdown, in the first position.
This is an example from Paul Stoffregen's SerialFlash library that copies data from microSD card to Flash memory. Here you learn how to use both types of memories at the same time.