I'm at the stage of designing a database for a small restaurant programm and cannot solve one problem. on the picture you can see the problem. i have 2 tables:

Products
Menu/Dishes

Dishes can consist of severeal products and products in turn can be a part of several dishes. how to implement this in my case? Should I create any intermediate table for it or something?

Thank you in advance!

Yes, you create an intermediate table. It's primary key will include the primary keys from your other two tables.

Blindman, I got it thank you for help!

Blindman, I got it thank you for help!

I would like to ask more in-depth this question:
in the intermediate table is it that the referencing rows MUST form a primary key, or can I create an "indipendent" primary key and just reference the rows without constraints to other tables? Actually I've already tried it and it seems to work, but I don't know if I'm missing anything. I need this to be able to leave one column "null" of three or four referenced tables. With my previous design postgres would retreive unwanted rows.
I hope I'm being clear enough.

No, there is no requirement that the foreign keys must form a composite primary key.
But you should enforce uniqueness on the pair, and a primary key is a simple method of achieving that. The other option is to create a unique non-clustered index or a constraint.

and what undesirable effects would have not enforcing uniqueness on the pair?

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haha, smart answer!