Thanks for checking again on this, Khaled.
I am accessing update definition the following way: Clicking on the feature layer from my AGOL content folder. Then, under layers, clicking the link to "Service URL." That takes me to the REST Services Directory, with the path Home > services > view_layer_name > feature_service_name. Then I change the URL to rest/admin/services, scroll down, and choose update definition. I'm not sure if that answers your questions about the view service vs view layer, but let me know if there's a better way to access.
Clearing my cache did not fix the issue.
My feature service was made through Survey123 Connect. Once we made view layers, we were not able to make schema changes through Connect. So we tried to add fields through the AGOL data tab, but the button to add a field didn't work. We also tried in Pro but it returned an error. Through testing we've found that the "Add Field" button turns on when the sourceSchemaChangesAllowed property is true, and disappears when it is set to false. We also have a test feature service made in a similar way that we have been testing on.
Thanks again,
Philip