I started learning Blueprints in Unreal Engine recently, I swear it has helped my problem solving ability in other general areas of programming. Watching the links light up as nodes are executed is pretty great for debugging.
d1 = n % 10
d2 = n/10 % 10
...
dk = n/10^(k-1) % 10
There are a number of implementations possible, iterative is probably more economical. Another way that avoids integer division (if you just want +/-/* ) is doing all your arithmetic in BCD (binary coded decimal), using 4 bits per digit.
I think those kinds of exercises are useful because there is some confusion around arithmetic. Sometimes there's confusion between what are numbers, and what are number encodings or digits (which themselves represent individual numbers). I found myself confusing (or simply not having the concept of differentiating) the digits of a number and a number itself. Say '14' to me was uniquely associated to those two digits. When you learn binary arithmetic, you start generalizing and see it could be written '1110' as well. The number 18 is a concept independent of its representation. So you can talk about the digits of a number: the digits represent individual numbers themselves, but they are taken together to represent another number. In my example, I had 'dk' and 'n', where n is a number that will usually be represented in binary form, but that is irrelevant, and 'dk' are their digits, as numbers, also usually represented in some binary form (again might or might not be relevant). You even consider simpler encodings such as unary (e.g. as used in tally marks or finger counting), those of course have lower efficiency (O(logn) vs O(n)). They're all just representations of this abstract concept that are numbers (with which we can do mathematics and arithmetic operations).
I think it's illuminating to distinguish between digital properties (properties specific to a digital representation), and properties of numbers. For example, 4 in base 3 is 11, which violates the property that even numbers are those whose last digit is even (only true for even bases). Divisibility by two is a numerical property, last even digit (implying evenness) is a digital property.
Numbers are so simple conceptually, it's their digital representation (and digital arithmetic) that's a bit more complicated.