Video 156
In this video I show how to set up a list (effectively a domain) in ArcGIS OnLine:
I create a new field
I create a list for that field
I populate the list
finally I add data by selecting options from my list

Lists are honoured in all your AGOL Apps so in a WebApp for example you will see your list appear.

You can publish domains from ArcGIS Pro to AGOL but ArcGIS Pro cannot edit those published domains (the domain gets converted into a list) you have to overwrite via a new service if you make changes to the domain in ArcGIS Pro (on a local gdb)

Your AGOL list will be honoured inside ArcGIS Pro.