Nothing happens without a cause. In modern science, each event can be attributed to a cause (more likely many causes). One can believe whatever one wants, but there are laws of nature.
Kamma is action by the mind, speech, and body done with intention (based on lobha, dosa, moha, alobha, adosa, amoha).
We do millions of such kamma a day, and most are not strong. But each one has consequences. Just like every action has a reaction in physics, each kamma has a consequence.
So, it is impossible to figure out how trillions of past kamma lead us to experience many types of sense inputs a day.
Only when we experience bodily pain (dukha) or bodily pleasure (sukha), those are due to strong past kamma. In other cases, some sense inputs are brought in and any suffering (or pleasure) that we experience is mind-made (somanassa/domanassa), also called “samphassa ja vedana”. Also see the topic:
Could bodily pain be due causes other than kamma vipaka?
All this is hard to explain in a post. As one learns Abhidhamma, one may be able to figure out. However, it is not necessary to fully grasp this complex issue. But it is not difficult to see the truth of the framework, based on its validity. It is the same as believing things that physicists have stated, even though one may not have studied physics and thus may not be able to comprehend the finer details. Of course, it is good to understand as mush as one can.
Just as in science, the complex theory of Abhidhamma can be proven wrong only if serious inconsistencies can be pointed out. That is how scientists throw away “bad theories” that cannot explain a new observation, and build faith in “good theories” as long as they are consistent with each new observations.