How do I collect a sims ashes from another household in the Sims 4?

There are a handful of different mods which help with this. I personally use this one, which lets me summon the Urns of any Sims who are ghosts to my currently selected Sim. I find that easier to manage than having to worry about which sim automatically got which urn. Using it just involves selecting the ghosts whose urns you want from a sim picker, super simple.

There are also mods that automatically drop urns into active sims personal inventories. I believe that is one of the functions which NPCC has, for example, and there are others.