The builder pattern is specifically used to build and Decorator to add special features post build.
e.g In the Pizza example above we can decide to use one of the two pattern based on the problem domain.
If Chilli Flakes are essential for a Pizza and similarly we have a lot of ingredients to add to Pizza out of which few are elementary to make the Pizza edible(meaningful state), we can prefer to use Builder.
Once the Pizza is constructed, we can later go and decorate it with different toppings like tomato sauce, olives, etc...
Also, it is stated above correctly that they can be used together but this will increase complexity. So we should use patterns wisely only when it is needed in the problem domain, else a simple construction will suffice.
