if the striker shoots and the keeper saves it, but the striker taps in the rebound, how is xG counted?

Two separate shots with two separate xG values.

is it counted twice or just once?

Twice, because there’s two shots.

what if neither shot goes in?

Nothing different, because if xG only counted shots which went in it wouldn’t be a very useful metric.

how exactly is xG counted? in some cases, it’s just confusing for me.

Here is an explanation of xG from a stats site. It may be useful.

FM more than FIFA but… Cherno Samba.