Hi there,
and welcome here.
Pay attention to the BSP (board support package) and weather your using the non-mbed or mbed versions on the BSP’s. We use the power profiler to collect the power Info you see.
The Power Ninja @msfujino 
is the most experienced with this topic hands down.
So I can only add ," what he said" 
take your time and read EVERYTHING you will get up to speed no problem. come here and post any questions,and code using the code tags above(</>) paste it in there, pictures hepl to, lots of very helpful smart people are present most days & nights 
I have used coin cells on Xiao no problem. I like these cr-2450’s
they are a little fatter…(thicker).
HTH
GL 
PJ
FYI, there are things like shutting down or sleeping the external Flash also will lower the sleep number. It’s great hardware and sometimes the software dev lags but the user base holds it down and parallel devs like adafruit and nordic add to the knowledge base.
What is your end vision with this? 
here is the latest from me…
Getting lower power consumption on Seeed XIAO nRF52840 - #59 by PJ_Glasso