Is it possible to give an everyday example of each for the pair kupować-kupić? Maybe it'll make more sense to me then.
This has been asked many many times on this forum. I did a quick search just now and found this thread:https://polishforums.com/archives/2005-2009/language/share-perfective-imperfective-verbs-17963/
If you do a search you will find more.
But to get you started:
imperfective - kupowałem chleb - I was buying bread - emphasis on the action of buying rather than the result (i.e. the reader doesn't know if I bought the bread in the end or not)
perfective - kupiłem chleb - I bought bread - emphasis on the result rather than the action.