Yes, good point.

This form int mx[n][n] is a special case of int *mx and both are linear memory!
But the first form offsets automatically contrast to the second where you have to compute offsets yourself.
So with this example:

    int mx[5][5] = {
        1, 11, 111, 1111, 11111,
        2, 22, 222, 2222, 22222,
        3, 33, 333, 3333, 33333,
        4, 44, 444, 4444, 44444,
        5, 55, 555, 5555, 55555
    };
    int *x = mx;

mx[2][3] is equivalent to *(x + 2 * 5 + 3)

And answering your question... int * is returned because you cannot return int[n][n] from the function!

"And what about the number 1010000000000000000000000000000000000000000000000000000000000000000 ?"

How often do you use that large number?

And what about float('inf')?

Just habit.

For function ptr func I think (*func)(...) is same as func(...) isn't it?

And what about the number 1010000000000000000000000000000000000000000000000000000000000000000 ?

Best practice is what people voted best practice. You must get used to it, and if you don't agree, you are free to vote for whatever you think is good practice.