Is it possible to save arcade expressions on AGOL so they can be picked (as default) and used again for various datasets, instead of needing to make them and saving them again for each and every dataset?
Currently not, but it would be a very good for ArcGIS Ideas . It would be great to have that functionality.
Within the same web map you can select an existing expression to reuse for another expression, but not store them as expressions accessible at all time.
Retrieving data ...