When a person sets Status of an Item to DONE I want to set “Completed By” Column to that person. This is not a “Last Updated By” although close.
We, at The Monday Man, have recently released to “beta” an app called “Bridges” that is a collection of Automation/Integration recipes. It includes this recipe that will do exactly what you are asking about:
You can find out more and apply to be included in the free beta here: The Monday Man Apps – for monday.com