Change the billing information form during purchase

[biciodark]

Hello everyone. I would like to know how I can solve the problem introduced by the new legislation on electronic invoices ( italy ) that obliges those who sell to request the code sdi and pec.
This is the problem: after adding an item in the cart, I will complete the purchase. So I have to complete (without registering) billing information. In this form I would like to add 3 option button: "Public Administration" "Company" and "Private", if i select the option button  "Public Administration" a custom field called "SDI" for example must be required. But if the "Private" option button i set, the "SDI" field must be NOT required.

What should I change to do this? Can it be done without putting the code in hand or is it necessary? if yes, what is the file to edit?
finally there is a plugin that allows me to do something like that?

The ideal thing would be to have 3 option button "PRIVATE", "Public Administration" and "COMPANY" and based on the choice to make fields appear rather than others.

VirtueMart 3.2.14 and Joomla! 3.8.7 Stable
Thanks in advance Fabrizio

[GJC Web Design]

you need to display and hide fields depending on the select choices?

The only way is to adapt the template and code what is required in JS/php

You can see I have done a similar functionality here:

https://www.4wdpartshop.com.au/my-account/editaddresscartBT.html#

it is the same form but different fields and validations are there depending on Personal or company

[biciodark]

Gjc thanks for replay. Yes i want to do this.
The only difference Is that i don't register the user but i compile
Only the billing information. Can you help me? Which file i must modify?
Thanks 

[GJC Web Design]

am available for hiring .. its extensive JS etc in   templates\xxx\html\com_virtuemart\user\edit_address_userfields.php

[biciodark]

Hi GJC in in   templates\xxx\html\com_virtuemart\user\edit_address_userfields.php i do not see the folder user.....i see only invoice.
where is the file that i modify?
thanks Biciodark

[jenkinhill]

All that means is that you do not yet have an override. The default version of the file to edit and publish in the position GJC advised can be found at components/com_virtuemart/views/user/tmpl/edit_address_userfields.php