in gdbms, there is the concept of "predefined joins" where you define a relationship edge that directly connects 2 nodes, i don't know how these guys do it, but at least in neo4j that's the concept

edit: just saw their blog, they ask you to define a "rel table" and that has to define all the joins and they load it from there

When you insert the Transfer records in my example, you indicate that they are "edges/relationships" between "Account" node records. The system interprets the values in those records implicitly as foreign keys to the Account records. This is what I mean by "predefined joins" in my example. When you ingest your relationship records, you predefine a join to the system and the GDBMS uses those Transfer records to join node records.

Hope this helps.