I think just like the other answers you should try your Domain Model first and see where are the pain points and improve from there. There is no perfect world.
However i have seen the below model in the past.
That way you can have good seggregation of duties between the entities. Then again this is 1 Dev’s View :)
Apply with caution :)
This depends on your Use Case.
Actually, many Customer can have many Items and 1 Orderdetails.
And one Item many Orderdetails.
Check out the Academy Courses for more Infos about Domain Models.
In my opinion, your diagram contains these mistakes:
1) Use only singular noums to identify your entities (i.e. Item) and Mendix best practices recomends you use a CamelCaps between nouns of your entities, use only Customer and Order to identify your entities, verify identifiers of your attributes too;
Verify more best practices at this link: https://docs.mendix.com/refguide/dev-best-practices/#321-entity-names
2) Item have a relationship only with Order, remove the relationship between Customer and Item;
3) The relantionship between Customer and Order is inverted, because one Customer can issue many orders