Fluid Form; form change according to dropdown selection

It’s a question of semantics as I don’t know js.
I’m trying to do a form that changes depending what fields are selected in dropdowns/checkboxes/etc. with semantic ui. What code do I use? I’d also want to not accept fields that are not rendered into db.

Also if using dropdown you get option value while with radio fields you get tabindex. Are those essentially the same?

Like in choosing something in radial menu as a general question it accordingly changes the theme of the form.

https://themeteorchef.com/snippets/reactive-dict-reactive-vars-and-session-variables/#tmc-reactive-dictionary

uses reactivedict to store several fields.