Hi there,
Ok to test the QSPI and Insatiate the QSPI interface look here at this example
worth a read to see the Nrf52840’s interfaces Here is the MeAT 
#define SS_SPI0 1 // seems to default to Pin 7 if you don't declare it as the CS for the Flash Chip A1/D1 pin1 on the Xiao
#define SS_SPI1 25
SPIClass SPI_2(NRF_SPIM0, PIN_QSPI_IO1, PIN_QSPI_SCK, PIN_QSPI_IO0); // Onboard QSPI Flash chip
Adafruit_FlashTransport_SPI QflashTransport(PIN_QSPI_CS, SPI_2); // CS for QSPI Flash
Adafruit_SPIFlash Qflash(&QflashTransport);
SPIClass SPI_1(NRF_SPIM3, D9, D8, D10); // MISO, SCK, MOSI /EXPANSION FLASH// CS pin D1 (hardwired)
Adafruit_FlashTransport_SPI EflashTransport(SS_SPI0, SPI_1); // Flash Type
Adafruit_SPIFlash Eflash(&EflashTransport);
HTH
GL 
PJ