I would like to know what is best way to add Enterprise keywords column into refinable string so it can pick up inside search
Go to search admin center and find out the OOTB
RefinableStringxx properties as below:
Click on Edit/Map property.
Then click on the Add a mapping.
After that, you can map it the Enterprise keywords as below:
Once this is done, wait for the crawl to get completed. This will populate the values in the refiner
After that, you can add this property in the Refinement webpart as below:
Once done, you can use it as below:
Reference - Enterprise keywords as refiners