Cat a/b +1mins, catb c +2 mins start times

Hi can someone help and tell me
how the different start times on zwiftpower affect the finishing times

Your start time is not influenced by other Cat start times - you should view other Cats with different start times as another race.

So what do the plus mins mean? For finishing times does it mean we add those mins to their finishing time

No, the finishing times are correct. The pluses are just telling you that each cat started a certain number of minutes after the official race start time.