TIDA-010270: Battery life calculation

Tool/software:

I want to know how to calculate battery life since I am new to this. In the design guide for TIDA-010270, the battery life calculations are given as below.

Can you please tell from where 830uA has come from? (I've highlighted by confusion with a question mark)

And how 265 hours is said as expected battery life?

Thank you,

V. Vedashree