I think it's sort of an illusion (though certainly a useful one) that numbers are all part of the same system. Multiplication of natural numbers is defined to be repeated addition, and I'm not really sure how you could define natural number multiplication in any other way. From the natural numbers you can go on to define the integers, rationals, then reals, and each has its own definition of multiplication, though they each depend on the definition of multiplication from the previous system.

There's a standard way of lifting each type of number to the next type, and this lift is compatible with all the basic operations (the lift is a "homomorphism"), so it's easy to pretend that the real numbers (or complex numbers if you want) are the universal system.

So with your example of going to fractional values, you're right, repeated addition falls apart -- but I'd say that's because it's not the definition for multiplication of rational numbers! Multiplying numerators and denominators is the usual definition, but that gives about as much intuition as does the definition for multiplying natural numbers. Sort of "the point" of multiplication of naturals, I think, is that it represents how many things you have if you arrange them in an n by m grid. Rational numbers show up in geometry with similar shapes (scaling), and for a few reasons you'd want multiplication to represent by how much something scales after a composition of scalings; maybe "the point" of rational number multiplication (at least algebraically) is that you can defer dividing until later, i.e. (a/b) * (c/d) is (a*c)/b / d.

Fractional numbers make sense to me as an extension, but it also requires an intuition of division on the same lines.

Take number n and multiply it by number x/y. To do this, you have to split number n into y parts and take x number of them. So to multiply 8 by 3/4, you split 8 into 4 parts (2 + 2 + 2 + 2) and then take 3 of those parts. This ends up being 2 + 2 + 2 = 6.

For multiplying two fractions, you have to extend it to n/m * x/y. Since you can multiply the top and bottom of a fraction by the same number, you can write n/m as ny/my. Then you can have n/my be your “equal part”, and take x of them. So 1/2 * 1/2, you take 2/4 and split it into 1/4 + 1/4 and 1 of them, so the answer is 1/4.

To me at least, this makes sense as an extension of multiplication is repeated addition. It’s when you get to irrationals that it starts to fall apart, and even then the intuitions the above way of thinking led me to have served me well.

As someone with an almost solely intuitive understanding of mathematics, I still find the "repeated addition" idea useful when it comes to fractions. If 4 * 4 is 4 repeated 4 times (4 + 4 + 4 + 4). Then 1/2 * 1/2 is a half repeated half times, or a half of a half, which is a quarter. The numbers get hard to work with but the intuitive idea is still there.