Gramps Data Model diagram (I did not understand the of relation in data, until I exploring by creating a diagram - Shared under Creative Commons Zero terms.)

IMHO showing all existing relations between the records is of little help in understanding the data model. Such a global drawing contains all potential links between records without explicitly telling in which circumstances these links are active.

I prefer to split the diagram into “local” diagrams related to primary interest records where only the links relevant to this record are depicted. See for example here the case of Person.

This is less confusing because it eliminates loops present in the global diagram.