We automatically detect the Buy Route once you add a Vendor.

We automatically detect the Manufacture Route once you add a Bill of Materials.

You can adjust Routes for Products as needed via the Replenishment Menu:

From the Odoo 19 Release Notes at https://www.odoo.com/odoo-19-release-notes

Product route configuration - Product routes are set automatically when possible (i.e., Buy route for purchase products and Manufacture route for products with bills of materials).