Hello,

That is shown when a field is defined as a payment widget(as widget="payment")

Below I've posted the compute function for the paymets_info field

@api.one
@api.depends('payment_move_line_ids.amount_residual')
def _get_payment_info_JSON(self):
    self.payments_widget = json.dumps(False)
    if self.payment_move_line_ids:
        info = {'title': _('Less Payment'), 'outstanding': False, 'content': []}
        currency_id = self.currency_id
        for payment in self.payment_move_line_ids:
            payment_currency_id = False
            if self.type in ('out_invoice', 'in_refund'):
                amount = sum([p.amount for p in payment.matched_debit_ids if p.debit_move_id in self.move_id.line_ids])
                amount_currency = sum([p.amount_currency for p in payment.matched_debit_ids if p.debit_move_id in self.move_id.line_ids])
                if payment.matched_debit_ids:
                    payment_currency_id = all([p.currency_id == payment.matched_debit_ids[0].currency_id for p in payment.matched_debit_ids]) and payment.matched_debit_ids[0].currency_id or False
            elif self.type in ('in_invoice', 'out_refund'):
                amount = sum([p.amount for p in payment.matched_credit_ids if p.credit_move_id in self.move_id.line_ids])
                amount_currency = sum([p.amount_currency for p in payment.matched_credit_ids if p.credit_move_id in self.move_id.line_ids])
                if payment.matched_credit_ids:
                    payment_currency_id = all([p.currency_id == payment.matched_credit_ids[0].currency_id for p in payment.matched_credit_ids]) and payment.matched_credit_ids[0].currency_id or False
            # get the payment value in invoice currency
            if payment_currency_id and payment_currency_id == self.currency_id:
                amount_to_show = amount_currency
            else:
                amount_to_show = payment.company_id.currency_id.with_context(date=payment.date).compute(amount, self.currency_id)
            if float_is_zero(amount_to_show, precision_rounding=self.currency_id.rounding):
                continue
            payment_ref = payment.move_id.name
            if payment.move_id.ref:
                payment_ref += ' (' + payment.move_id.ref + ')'
            info['content'].append({
                'name': payment.name,
                'journal_name': payment.journal_id.name,
                'amount': amount_to_show,
                'currency': currency_id.symbol,
                'digits': [69, currency_id.decimal_places],
                'position': currency_id.position,
                'date': payment.date,
                'payment_id': payment.id,
                'move_id': payment.move_id.id,
                'ref': payment_ref,
            })
        self.payments_widget = json.dumps(info)

You can add any field you want to it by changing real code or inheriting it and add value to content

Also you should change account_payment_widget.js and its corresponding qwebview(account_payment.xml)