A widget is the most fundamental building block of the UI in UMG. Every element is a widget, you can combine multiple native (button, border, grid panel) widgets to create more complex User widgets. You can then combine user widgets with more user widgets to create even more complex systems. Each widget can be animated.
You can use materials with widgets to produce more advanced effects. And you can use a Retainer Box to push things a bit further:
Anchors will allow you to deal with most of it. There are also scaling boxes:
a UI that has effects inside(emissive sprites, particles, and so on
Not something that is fully supported out of the box - here, the Niagara plugin comes in handy.