In Odoo, the product field in the invoice line items is designed to display product variants by default. The reason behind this is that each variant represents a unique combination of attributes (such as color, size, etc.), which may have different prices and stock levels. This allows for more accurate invoicing and inventory management.

However, if you’d like to see all of your products when creating an invoice, you can follow these steps:

1. Enable Product Templates: Go to Inventory > Configuration > Settings. Under the “Products” section, enable the option “Product Variants.” Save your changes.

2. Update Product Templates: Go to the Products menu in either the Inventory or Sales apps, click on each product template, and make sure you’ve created at least one variant for it.

3. Select Products in Invoice Line Items: Now when you create an invoice and add line items, you should be able to select from all of your products (including templates with multiple variants). Just start typing the name of a product or browse through the list.

Please note that enabling product templates will affect other areas of Odoo as well (e.g., sales orders, and purchase orders), so make sure this change aligns with your business requirements.

I hope this helps! Let me know if you have any further questions or need additional guidance.