How to find union of nodes in rdflib knowledge graph

Little background on Work : I am working with ontologies and for my usecase I have to apply random walk on the ontology nodes/entities. In order to do the same I have written one function - that given a node it will output all its immediate neighbour nodes.

Actual Problem : Recently I came across an ontology which along with normal nodes in the graph also has union of other nodes as an entity. But while going over the triple's of this ontology I am unable to find any triple which mentions such union operation.

Please refer this ontology - https://service.tib.eu/webvowl/#goodrelations

I want to know if there exists an URI for such special union nodes or by any way we can know which nodes form an union.

PS : You can see the union entity highlighted in red circle below.