I want to open an options menu from another button, not from an actionbar button, like so:
How can I make a menu show up on a button like this?
...
What you're looking for can be done using a PopupMenu
object.
You can look at these simple examples:
https://www.tutorialspoint.com/android-popup-menu-example
or
https://www.javatpoint.com/android-popup-menu-example
There are many other examples on the Web and YouTube.