Answer 1

the diference is in the declaration:
int [] x;
int [][] y;

dimensions

int [] x = new int[3];
x.length = 3;
int [] y = new int[3][3];
y.length = 9;

In the search indexes values or fill indexes values
x[0] = 2;
y[0][0] = 5;

arrays 1d contains rows of datas
arrays 2d contains rows and columns as a table of datas