help to understand this please

Question

int[ ][ ] myArr = { {1, 2, 3}, {4}, {5, 6, 7} };
myArr[0][2] = 42;
int x = myArr[1][0]; // 4

my Question is why, what and how is this true ?
myArr[0][2] = 42;

Answer 1

Why did the course did not explain this ?
How could i know by my self that myArr[0][2] = 42 means overrides the previous value 3 and becomes 42 ?
why is this course so bad and poor.
can you please go throw your course again and do it better!