Which way to open a menu with a button?

Hello,

it’s my first time with Kotlin, so i try to create a life counter for magic the gathering in Android Studio.

My problem is, that I don’t know how i can use a button for opening a menu. So I designed two xml files and now I want the button on the first picture on the bottom end of the grey part to open the menu (second picture). But do I use fragments or the menu bar or just open another activity on top?