If you need to change markup then you can create a custom forms theme.
https://docs.umbraco.com/umbraco-forms/developer/themes
If you just want to restyle you can just override the css on the front end.
Most of the forms markup is easy enough to style - I had to build some custom data types to have custom checkboxes so I had more control on the label tags.
Or am I misunderstanding what you're asking?