Answer 1

If you name an element in your HTML document using the id attribute, and if the Window object does not already have a property by that name then the id is created by default as property of global object , and  the Window object is given a property whose name is the value of the id .
That's the reason why you can use the id without using getElementById

Answer 2

click.innerHTML

Answer 3

IDs of elements are registered as global variable. Some say this was Chrome specific, so I'm not too sure whether all browsers shows such behaviour.

https://dev.to/vuelancer/dom-element-id-as-global-variable-1l2n
https://2ality.com/2012/08/ids-are-global.html

Answer 4

Cuz you have used onclick

https://www.w3schools.com/jsref/tryit.asp?filename=tryjsref_onclick

https://www.w3schools.com/jsref/event_onclick.asp