You can use either one of these to shutdown the chip:
sd_power_system_off();
NRF_POWER->SYSTEMOFF = 1;
But to wake it up, you need to cycle the power to the chip (BLE and all peripherals). Also, in the case of the XIAO BLE Sense that has the IMU and the Mic circuits…these will remain on using about 2mA even as you shut the chip down. So you need to find the pin that supplies power to the IMU and the MIC and have it turned off in the code before you invoke these.
Or use a circuit like this that you can control either manually or with a digital pin. Still requires user physical action to restart the chip.Pololu - Mini MOSFET Slide Switch with Reverse Voltage Protection, LV