I understand that the future perfective is basically the present tense with the perfective prefixes. But what happens if it's a non-prefix verb I.e. kupować/kupić? It's there any general rule/ explanation? Thanks :)
No, I wouldn't say that it's Present Tense with a prefix. For example here, "powiem" (I will tell) sounds nothing like "mówię" (I am telling). For a learner, Future Simple and Present Tense may be very similar, because generally the verbs in Future Simple can look like they are Present Tense. It's exactly the fact that they are perfective (and therefore it's logically impossible to use them in Present Tense) that makes it Future.
So when you're learning Polish and you see "kupuję" and "kupię", it's not like one of them strikes you as Future Tense, right? You have to see that "kupuję" is from imperfective "kupować" and "kupię" is from perfective "kupić".