Is there a way in Figma to add constraints to shapes inside a union. For example I need to create a modal window with an arrow that keeps it position on resize. Or is there an other way to do this?
Yes, you need to wrap the union in a frame, then you will be able to assign constraints to elements inside of it. Constraints can only be set on objects inside frames, and groups/boolean operation objects are ignored, the objects inside of them will stick to the outer frame.