In Adalo App Builder (database contents editor modal) you can select the “parent” item in a 1:many relationship. E.g. when a Product belongs to a Category, you can open a product record and select the category from the list. You can not select several products from a Category record to set a relationship.
In many:many relationship you can select multiple entries in the relationship field in the database editor modal.
What @dilon_perera probably mentioned is setting up the relationships upon CSV import. In the link above, you need to pay specific attention to point 3 of the manual, especially to the explanation how to set up CSV and the collections so that relationships will be recognised automatically.
The alternative option of setting the relationship automatically would be running an API call in some script to update the relationships with it. However, this require some advanced knowledge.
In my opinion, if you have 20-30-50 restaurants, it’s easier to do it by hand
@Victor thank you for the detailed explanation, my problem in linking CSV file was that I had Uppercase naming in Adalo and lowercase naming in CSV… Thanks once again!