Hello,
so i have to manage price and quantity in varients section tab (right?)
Yes, if you want to let the price of your main product set to 0, then if you want your customers to be able to add your product variants, then you'll have to add a price to your variants.
but the problem is if a varient quantity = 0 the customers still can add the product with varient to cart with price of main product (not varient price)
If you have set your variant quantity to 0 then you shouldn't be able to add it on your cart, are you sure that you didn't added to your cart another variant of your product ? If that's the case then can you show me some screenshots of how you configured your product and its variants ?