for me, it’s pretty confusing in quite a few instances, but, just to give an example: if the striker shoots and the keeper saves it, but the striker taps in the rebound, how is xG counted? is it counted twice or just once? or, what if neither shot goes in?
Two separate shots with two separate xG values.
Twice, because there’s two shots.
Nothing different, because if xG only counted shots which went in it wouldn’t be a very useful metric.
Here is an explanation of xG from a stats site. It may be useful.