Hi,

To display the "Expected Arrival" field from the Purchase Order model in the "Product" page of the Product Template model.
First of all, Inherit the Product Template model and define a related field in the Product Template model that will fetch the "Expected Arrival" value from the related Purchase Order.
Write a compute function to fetch the most recent Purchase Order for the current product and set the value of the expected_arrival field accordingly. as given below

class ProductTemplate(models.Model):
    _inherit = 'product.template'

    expected_arrival = fields.Date(
        string='Expected Arrival',
compute='_compute_expected_arrival',
        store=True,
        readonly=True
    )

def _compute_expected_arrival(self):
        for product in self:
variant_ids = product.product_variant_ids.mapped('id')
purchase_order = self.env['purchase.order'].search([
                ('order_line.product_id', 'in', variant_ids)
            ], order='date_order desc', limit=1)

            if purchase_order:
product.expected_arrival = purchase_order.expected_arrival_date
            else:
                product.expected_arrival = False
               
  Update the XML view also,
               

<record id="view_product_template_inherit" model="ir.ui.view">
<field name="name">product.template.inherit</field>
<field name="model">product.template</field>
<field name="inherit_id" ref="product.product_template_only_form_view"/>
    <field name="arch" type="xml">
        <page name="purchase" position="after">
            <group>
                <field name="expected_arrival"/>
            </group>
        </page>
    </field>
</record>

Hope it helps