When I am designing at 1x resolution, can I extract assets from smart objects or linked smart objects for 2x or more? If so, how do they scale up?
Yes with caveats.
PSD and Illustrator smart objects are also re-rendered and smoothly scaled to the requested resolution.
Other smart objects (including those with filters) are only scaled up from the resolution they are at within the Photoshop file. If you want them to re-render, you can paste them at 100% resolution into a linked smart object and get the smooth scaling.