কনসেপ্ট
First, you should read the code two times without comment.
I think, you will understand the algorithm. if not, then follow comments.
————————————-
Here, we use primary school math logic. let two numbers $1133$ and $333$.
$1133$
$333$
———-
3399 ->3399 ….. res[0] = 9
33990 ->3738[9] // add 3399 + 33990 = 37389 ….. res[1] = 8
339900 ->3772[8][9] // add 37389 + 339900 = 377289 … res[2] = 2
———-
377289
on first line we took $3$ and multiply with $1133$ and result is $3399$
on second line we took $3$ and multiply with $1133$, and we got 33990,
then we add this $33990$ with $3399$ and result is $3738[9]$.
on third line, we follow same way.
you should read code first to know res[0], res[1], res[2]…
—————————————