Your problem stems from the monotonic increasing interval of sinx. As long as you understand this problem, you will understand it.

First of all, we say that the image of y=sinx is a wavy line, that is, it is not monotonous throughout R, and it can be said that it is from maximum to minimum.

Monotonically decreasing, from the minimum value to the maximum value, we can say that the function monotonically increases.

There is no 2kπ, only the monotone interval in a certain week is found, and the whole R has countless weeks. After adding 2kπ, all monotone increasing intervals in a week are found.