You’ve already gotten some good answers, but I just want to point out WHY your uncle’s problem makes no sense: what does it mean to multiply an apple by an apple? Like physically, what do you *do* to multiply them? It’s a nonsense question.
The most rudimentary way to understand multiplication is as repeated addition, so for example if a sack of apples has 10 apples and you have 3 sacks, you can find the total number of apples by taking 10+10+10 or 3*10, the units being apples per sack times sack, coming out as apples.
In another example, if we wanted to find the area of a rectangle that’s 2 inches by 6 inches, we could count up the number of 1inx1in squares by doing 6+6 or 2*6, because what we’re counting is 1x1 squares, the units are in square inches.
But it doesn’t make sense in the real world to multiply apples by apples, there’s no corresponding physical situation. If we were talking about an abstract mathematical situation this wouldn’t be a problem, but we’re talking about apples. Your uncle’s “major problem in mathematics” is his misunderstanding of what it means to multiply things.