Jul 24, 2019 09:40 AM|Yang Shen|LINK
About this question, i suggest you can learn about the page life cycle of webform. Please refer Life Cycle.
In the page life cycle of webform, init event occurs before load event.
Init event is initialization step that the server control should perform to create and set instances.
View state information cannot be used within this event, it is not populated yet.
A load event is used to load the relevant data into the corresponding control, and view state is available.
For what is Oninit(EventArgs e), please refer to:Here.
For the difference between Oninit and Pageload, Please refer to Here and Here.
The "base.OnInit(e)" in Oninit() is to ensure the parent class executes the Page_Load first.