Hi,

It seems you are new to Odoo development. If you are going to build the website for your company either you can do it from the user interface using the odoo website builder or if you are technically sounded you can create a new module to add and design the website accordingly.

Odoo has got a module structure, if you check the odoo github repository, you can see lot of modules in it, each of the module adds a set of functionality to the system

Github: https://github.com/odoo/odoo

And by checking the odoo code you can see how the menu is added in the website, see a sample code of how the shop menu in the Ecomerce module is added:- 

<record id="menu_shop" model="website.menu">
    <field name="name">Shop</field>
    <field name="url">/shop</field>
    <field name="parent_id" ref="website.main_menu"/>
    <field name="sequence" type="int">20</field>
</record>

See the above code here: https://github.com/odoo/odoo/blob/13.0/addons/website_sale/data/data.xml

Also if you are going into development, you can set a development environment, then it will be easy to check the codes. Better to configure Odoo with Pycharm.

Odoo Development Tutorials: Odoo Development Tutorials

Thanks