"XMLFile"

<form>
                    <sheet>
                        <h1 align="center">Fatora</h1>
                        <group>
                            <field name="name" string="client name"/>
                            <field name="bill_number" string="Number"/>
                            <field name="bill_date" string="Purchase date"/>
                            <field name="Product_id" string="products" widget="many2many_tags"/>
                        </group>
                    </sheet>
                </form>

model market.bills.py

class PurchaseBill(models.Model): 

_name = 

_rec_name = 

bill_number = fields.Integer() 

Product_id = fields.Many2many('market.products')  

bill_date = fields.Datetime() 

client = fields.One2many('market.clients','bills')

model market.clients.py

class MarketClients(models.Model):
    _name = 'market.clients'

    name = fields.Char()
    bills = fields.Many2one('market.bills', string='client bills')