If you're using one of the Avonni Components on your Lightning web component, Avonni will ask you to install the Avonni Components package. The Avonni Components are free and it's an open source project, released under the permissable MIT license. They can be used in personal or commercial projects for free.
If your LWC (with Avonni components inside) is not working on Salesforce once deployed, please make sure to activate Lightning Web Security on your org.
Source: Enable Lightning Web Security in an Org (from Salesforce)
View all the differences between Avonni Designer and Avonni Builder.