Holly - thank you for your time and your response. Thank you for providing the link to the ESRI article.
I used a relationship class developed in ArcGIS Pro to bring the related records from table B into the pop-up menu for layer A in AGOL. My question has one more twist: can I show the related records from table B in the pop-up menu for layer C? The idea is that I could access the polygons from layer A which intersect the user-selected polygon in layer C, and then show the related records from table B in the pop-up menu for layer C.
I am not sure if this is possible, because it seems like related records can only be tied to the layer that has the relationship class. In this case, layer A has a one-many relationship class with table B, while layer C is a separate layer.
I have been testing whether Arcade functionality (namely $feature, Intersects(), and FeatureSet functions) can be used to grab all the polygons from layer A that intersect the user-selected polygon from layer C, and then show the related records for the polygons from layer B in the pop-up menu for layer C. Using the example geographies above, in the pop-up menu for an individual state (layer C), I want to show all the related records (table B) for all the counties (layer A) and intersect the user-selected state (layer C).