One thing that’s quite different form sitemaps is MainUI doesn’t really have a button as a part of a widget. Instead the whole widget is the button. So if you want a button, you will use an oh-label widget inside another widget and set the action to do what you want to do, or set the icon or the like. Pages - Custom Widgets | openHAB shows a little more complicated than probably what you are after but if you focus on the “Stop” part of the widget you’ll kind of see what I’m saying.
Ultimately, make the widget look like you want it, set the action and you’ve got a button. Again, there isn’t a concept of a button as a separate thing.
If I understand correctly, you are wanting to put this into the cards on the autogenerated parts of the Overview page. The way to do that is to set the “Default list item widget” Item metadata. So you can construct. a widget that looks and works how you want, set that as the default list item widget, and that widget will be used in the location/equipment/properties cards.
As for the pop-up, that’s just one of the many Actions that can be done when the widget is clicked on.
An example widget that I built for that may be the one on the marketplace that you are referring to (I think I’m the only one who has published garage door opener widgets on the marketplace. Garagedoor
In this case, the whole widget is the button which, when clicked sends a command to a Switch Item that triggers the garage door opener. The icon on the left and badge on the right is driven by a Contact sensor Item indicating the open/closed state of the door.