A Prototype Project gives you all the flexibility to create your prototype the way you want without any constraints from Salesforce. A prototype Project is not deployable on Salesforce.
It’s all about creating Lightning web components visually. An Avonni Builder project will give you the ability to design, prototype and build a working Lightning web component that you can deploy on your Salesforce Org with the click of a button.
View all the differences between Avonni Designer and Avonni Builder.