Hello,
you can search the report in settings>Technical(visible in debug mode)>Reporting> Reports,
change report type to HTML
select Qweb views(smart button) make changes, then print the report, make changes again, rinse repeat
don't forget to copy changes to your code and upgrade module.
this is an easier method than making changes and upgrading each time.

Hello,

you can follow the example of this template which inherits the original invoice template and adds the fiscalcode

<odoo>
<template id="report_invoice_document_fc" inherit_id="account.report_invoice_document">
    <xpath expr="//span[@t-if='o.partner_id.vat']" position="after">
        <span t-if="o.partner_id.fiscalcode">Fiscal code: <span t-field="o.partner_id.fiscalcode"/></span>
    </xpath>
</template>
</odoo>