How to make a button?

How would I make an object that reacts to an object on top of it to, maybe open a door, or something else?

You can create a button using the user interface playcanvas or HTML / CSS. After that you can handle the event of pressing the button and trigger the necessary action for entity of the door.

Do you mean a floor button or switch like that you have in Portal?

yeah i need something like that from portal