In the source solution look for MVC -> Kentico.Content.Web.Mvc.Views -> Views -> Shared -> Kentico -> FormBuilder | FormComponents.
I believe if you re-create Views under the same path (
~/Views/Shared/Kentico/...) with the same name in your application, MVC will use your files instead of the ones included in the Razor class library.
For the Form Builder configuration, there are 2 parts.
CMS_Form table there is the
FormBuilderLayout column that defines how the Form Builder UI is structured for a given form. This is serialized JSON.
CMS_Class table there is the
ClassFormDefinition column which is all the configuration for the Form that gets rendered on the live site, which you can find for all your forms with this query:
where ClassTableName like 'Form_%'
This column is serialized XML.