If any report have a combination of two elements,like below->
1-in reporting all session contain an ir.actions.report.xml, for which a <report> shortcut element is provided, for example code is below-->

<report id="account_invoices" model="account.invoice" string="Invoices" report_type="qweb-pdf" name="account.report_invoice" file="account.report_invoice" attachment_use="True" attachment="(object.state in ('open','paid')) and
        ('INV'+(object.number or '').replace('/','')+'.pdf')"></report>

2- And other contain a standard QWeb view for the actual report.
for example code is below->

<t t-call="report.html_container">
    <t t-foreach="docs" t-as="o">
        <t t-call="report.external_layout">
            <div class="page">
                <h>Report title</h>
            </div>
        </t>
    </t>
</t>