I did create my first customized Contact form with Form Builder. However, I am unable to create a Membership form: currently le proposed field sets are limited to «Individual» «Activities» «Organization» and «household». Am I missing something or is it merely the current state of the project ?
Welcome to SE! Currently Membership is not supported in Form Builder. A prerequisite for Form Builder is that the entity is supported in APIv4 - and it isn't yet. See https://lab.civicrm.org/dev/core/-/issues/2023