What is different between array 1d and 2d

Answer 1

int[] arr = new int[5];
this is one dimensional array you have only rows. number of rows is 5.

int[] array = new int[5][6]
this is 2d array you have rows as well as columes.
number of rows is 5.
number of columes is 6.

Answer 2

no different. it's the same for your memory allocation
we can see a 2d array as an 1d ..