How does supply planning create a link between item structure and work definition

Summary:

We are collecting data from EBS system thru custom queries and FBDI. There isn't a 1 to 1 relationship between BOM and routings (neither in EBS nor in cloud - although cloud work definition allows you to pick an item structure) and We do not see any columns in the two FBDIs that would allow us to knowing link the 2. But at the same time we see thru plan inputs of item structure that it does have both item structure name and work definition name (along with the attribute called Primary). How does it do that ?

Ideally, we would prefer to create that relationship ourselves