Hi Phil,

For setting variants for products on your website using Odoo, you can refer to the following documentation:

Setting Product Variants on Odoo Website

This documentation provides detailed instructions on how to configure and manage product variants within Odoo's sales module. It covers topics such as creating product variants, defining attributes, setting prices for variants. By following this guide, you can effectively set up and manage product variants to offer a diverse range of options to your customers on your Odoo website

Hope it helps

+1 I need to set default variant in product page like selected when client open the product page !

There is no configurable option - as per my knowledge (without customizing)
But following patch you could try if it works for your case
- changing the sequence of Variant Attribute Values