Relaptionship betwen tabeles/collection

I’m having trouble handling the relationship between Strapi tables.
For example: foreign key, when creating tables from timo N to N “Many to Many” Strapi creates the 3rd table of the relation but does not create constraints and does not show the table at webAdmin.
If necessery to add more information in this 3rd table, It must be done directly in the database.
Do you have any solution in this case?

Another question is about Graphql, I can’t make a single Mutation to insert into 2 tables, for example:
Product: banana
Category: fruit

Suppose I want to insert both values into the tables, none of the records exist. How I can create a mutation to insert both values?
Thank you very much for your attention.