Hi there,
Pfffft Childs play on the xiao 2uA on sleep. No MCU goes lower than the Nordic nRF52840 so
Read some data sheets and see what it is your needing to do in Black and white.
Xiao could be sleep always(lowest power) and Button push could be Wakeup interrupt to send BLE button press, back to sleep… LOL! you won’t get anything more power efficient at all. 
HTH
GL 
PJ
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