How to put button in the corner of the screen?

Thank you, I’ll take a look at the list of the methods.
The screenshot:

As you can see I added a small circle in the corner that represents the button. The button should float in air (meaning if I scroll down, it should still be in the corner). The logic that will be executed once clicked, I will try to add myself but to make it more clear, currently I’m trying to understand how it will open an alert box (just for practice).