The manual: https://docs.qgis.org/3.16/en/docs/user_manual/working_with_vector/attribute_table.html#creating-one-or-many-to-many-relations describes how to create a N:M relation and how to use it with a attribute form made with drag and drop designer.
With Qt Designer I am only able to create a form for a 1:N relation using a
QgsRelationEditorWidget and set the qgisRelation property accordingly. But how do I use a
QgsRelationEditorWidget for a N:M (many to many) relation?