Hi,
To enable alternative products, go to Website ‣ eCommerce ‣ Products, select a product, go to the Sales tab, and enter the products you wish to feature in the Alternative Products field. Then, go to the related product page by clicking Go To Website, and click Edit. Stay on the Blocks tab, and scroll down to the Dynamic Content section. Then, drag and drop the Products building block anywhere on the product page.

When placed, in Edit mode, click the block to access various settings for that Products building block. In the Filter field, select Alternative Products. You can configure several additional settings, such as how many elements are displayed (Fetched Elements), the Template used, etc

refer this official documentation from odoo:

https://www.odoo.com/documentation/16.0/applications/websites/ecommerce/managing_products/cross_upselling.html#:~:text=To%20enable%20alternative%20products%2C%20go,To%20Website%2C%20and%20click%20Edit.

hope this will help you
thanks