numpy - explanation of boolean indexing behaviors

Question

For the 2D array y:

y = np.arange(20).reshape(5,4)
---
[[ 0  1  2  3]
 [ 4  5  6  7]
 [ 8  9 10 11]
 [12 13 14 15]
 [16 17 18 19]]

All indexi