the button on the left is the reset button… same as the reset button on the board… it was made for the original SAMD that did not have a reset button on board the user button is on D1
if you are using the expansion board you can use Grove - Dual Button - Seeed Studio
If you read this part about strapping pins and understand you may be able to get the boot button to function as a user button, but the reset pin as i understand pulls directly to ground in hardware